The knowledge of present and past sea surface temperatures (SST) is of general importance for climate modelling because the SST controls climatic factors like heat exchange and humidity at the interface between ocean and atmosphere. Here we present comparison of measured SST with Sr/Ca and U/Ca ratios. The objective is to assess the potential of Sr/Ca and U/Ca for reconstruction of past SST variations. In order to examine the potential for decadal and centennial SST reconstruction's in the Indian Ocean we studied the upper part of Porites lutea cores with high resolution sampling (monthly resolution) and compared the results to instrumentally recorded temperature data sets. The corals used in this study derive from different sites in the Indian Ocean, from La Réunion Island, the Ningaloo Reef and the Abrolhos Islands. The La Réunion and Ningaloo Reef corals represent the tropical Indian Ocean with SST´s in the range of 22 to 29°C whereas the Abrolhos Island corals grow in cooler water with 19 to 25°C. The SST - Sr/Ca- and U/Ca relationships are established by calibrating Sr/Ca and U/Ca ratios to available Reynolds data sets (Reynolds et al., 1994). The isotope ratios are determined on a Finnigan MAT 262 RPQ+ mass spectrometer. External reproducibility for Sr/Ca and U/Ca measurements amount to ±0.2‰ (2<sigma>) and ±1.8% (2<sigma>).
Calculation of SST´s (SSTmeasured - SSTSr/Ca or U/Ca) showed that recent temperatures can be reconstructed in most cases with a precision better than 1°C. Calculated SST´s for the maximum and minimum values show higher statistical precision. However, there are some larger discrepancies to the equations published by other authors (deVilliers et al., 1994; Shen et al., 1996; Min et al., 1995), that may reflect different coral growth rates, elemental fractionation processes during skeletal formation by biological controls and spatial variations of the Sr/Ca and U/Ca sea water ratios. In addition, interlaboratory calibration of spikes and methods may also be required in the future.
One reason for systematic deviations of the measured Sr/Ca data from the linear relationship between SST and Sr/Ca are due to differential changes of the growth rates. We are able to reconstruct these non linear variations of the growth rate with high time resolution.
The reconstruction of Sr/Ca- and U/Ca-paleotemperatures of Holocene corals (U/Th age about 5 ka) from the Ningaloo Reef show good agreement for most of the samples. Both, Sr/Ca and U/Ca thermometer show consistent results for Holocene corals from the Ningaloo Reef. The mean annual SST estimated from both thermometers is about 22°C. Measured modern average SST is 25.7°C (Reynolds et al., 1994).
Reynolds et al, Internet: http://ferret. wrc. noaa. gov/fbin/climate_server, (1994).
deVilliers et al, Geochim. Cosmochim. Acta, 58, 197-208, (1994).
Shen et al, Geochim. Cosmochim. Acta, 60, 3849-3858, (1996).
Min et al, Geochim. Cosmochim. Acta, 59, 2025-2042, (1995).
TIMS measurements of Sr/Ca and U/Ca ratios have been performed on a massive Porites sample from isotopic stage 5e (dated at 131 - 132 kyr BP by TIMS U/Th) (Cabioch et al., 1996) and from post-glacial reefs cored in New-Caledonia (dated at 5690 and 5930 years BP by 14C) (Lecolle & Cabioch ,1988). Temperature calibrations have been attempted on two modern massive colonies of Porites lobata collected in the vicinity of the drill hole. These two colonies give different records for Sr/Ca and U/Ca ratios, as well as for 18O and 13C. The difference is largest for U/Ca. The discrepancies cannot be related to variations in calcification rates nor in sea-water chemistry.The records obtained in the fossils corals are clearly noiser than the modern, with only a poor seasonal cyclicity. Paleotemperatures estimated from the Sr/Ca calibrations indicate that the pleistocene Porites grew in sea water colder than today by 1.9 - 2.5°C, with a similar annual amplitude of 4.2 - 5.4°C, compared to 5°C in modern. U/Ca would indicate a temperature colder by 3.7 - 4.7°C with an annual range of 4.5°C. Last interglacial temperatures colder than present have also been reported by McCulloch et al. (1994) in Papua New Guinea (- 6°C for a Porites dated at 124 Kyr BP). These results are thus in contradiction with other proxies suggesting SST similar to present during the interglacial period (Crowley & Kim, 1994). While all the studied fossil Porites are devoid of obvious diagenetic perturbation on X ray diffraction spectra, SEM pictures of the New-Caledonia pleistocene and holocene Porites clearly show the presence of secondary aragonite cements. This diagenesis is not uniform, with clear variations at the micrometric scale. The diagenetic perturbation seems to be most sensitive for U/Ca: one of the holocene samples, which shows the most extensive precipitation of secondary aragonite, would indicate U/Ca derived SST colder than today (- 4.5°C), and an annual range of almost 17°C. However, these results suggest that Sr/Ca records may also be altered, and that colder Sr/Ca temperatures found in most post-glacial and in pleistocene corals analyzed to date, must be considered with caution.
Cabioch G, Récy J, Jouannic C & Turpin L, Bull. Soc. Geol. France, 167, 729-742, (1996).
Lecolle JF & Cabioch G, Mar. Geol., 81, 241-260, (1988).
McCulloch MT, East T, Mortimer G, Li XH & Chivas A, EOS (Trans. Amer. Geophys. Un.), 75, 333, (1994).
Crowley TJ & Kim KY, Science, 265, 1566-1568, (1994).
Information on the hydrographic properties of surface ocean waters during glacial and interglacial conditions is crucially important for understanding past climate change. Sea surface palaeotemperatures (SSTs) have traditionally been estimated from faunal abundances and, more recently, from alkenones in sediments. In principle, SST also may be obtained from the oxygen isotopic composition of planktonic foraminiferal calcite after correction for ice volume effects but this approach can be complicated by local salinity variations, vital effects and diagenesis. Results from inorganic studies have long suggested that Mg/Ca in carbonate minerals is mainly controlled by temperature. More recently, Mg/Ca data from ostracods, corals, and cultured and core top planktonic foraminifera, have suggested that Mg partitioning into biogenic carbonates is also temperature dependent. We present Mg/Ca and oxygen isotopes for several different planktonic species from coretop calibrations in the North Atlantic using NEAPACC and ANAP cores. Ancillary electron probe analyses are used to distinguish calcite with different Mg/Ca ratios.
We present two ca. 270 ka paleo-SST records from the eastern equatorial Atlantic derived from Mg/Ca-ratios in the planktic foraminifer Globigerinoides sacculifer. The downcore Mg/Ca-variations were calibrated using specimens from core-tops and grown under defined laboratory conditions.
The experiments suggest that physiological processes driving the foraminiferal magnesium uptake are a function of sea water temperature and that small-scale salinity changes have no impact on the Mg/Ca ratios, indicating that global ice volume changes, river-runoff, and evaporation/precipitation differences do not perturb the chemical signature.
Secondary calcite generated prior to gametogenesis exhibits significantly higher Mg/Ca ratios than primary calcite. Since calcite enriched in magnesium easily dissolves in the marine environment and is not well preserved downcore, we applied the species-specific, exponential Mg/SST relationship established for primary calcite to the downcore Mg/Ca-record.
The comparison of paleo-SST calculated from Mg/Ca ratios for Holocene samples with both temperature estimates from the World Ocean Atlas (Levitus, 1994) and the depth habitat of G. sacculifer suggests that the chemical signature must have been preferentially generated during austral late fall/early winter. The Mg/Ca derived SST estimates thus seem to reflect seasonal sea surface temperature conditions from the surface mixed layer.
Both records show distinct glacial/interglacial variations in Mg/Ca ratios with enhanced ratios during warm periods. Within the Southern Equatorial Current, paleo-SST vary between ca. 24°C and 27.5°C, whereas the Equatorial Divergence exhibits SST of only 22°C to 25.5°C. Common to both records is a glacial/interglacial amplitude of ca. 3-3.5°C for the last climatic changes, consistent with paleotemperature estimates from unsaturated alkenones. Moreover, both records imply lower Holocene and glacial stage 2 temperatures compared to the Eemian and glacial stage 6 temperatures, respectively. These results contradict paleo-temperature estimates from faunal analyses and isotopic measurements, which suggest larger glacial/interglacial temperature contrasts and warmer Holocene and glacial stage 2 conditions in comparison to the Eemian and glacial stage 6.
In order to reassess the relationships between past sea surface temperatures (SST), sea surface salinities (SSS) and global sea levels (SL), we analyzed alkenones in core SU8118 off the southern tip of the Iberian margin (38N, 10W). The last deglaciation section of this core had been dated precisely by means of AMS-C14 ages (Bard et al. 1987) which can be converted into calendar ages by means of calibration formula based on TIMS-230Th dating in corals (Bard 1998). The UK37'-SST record exhibits abrupt temperature changes in phase with 18O variations measured in planktonic foraminifera. The timing of these paleotemperature changes is in general agreement with those reconstructed by using foraminifera transfer functions (TF and MAT). While the holocene SST remained roughly stable at about 18-19°C, strong SST minima between 10 and 12°C are recorded at about 25,000, 23,000, 17,000 and 12,000 cal-yr-BP. An abrupt 3-4°C temperature rise takes place at about 15,000 cal-yr-BP marking the onset of the Bolling period. The Younger Dryas cold event is well defined starting with a 3-4°C cooling at the end of the Allerod interstadial. The Younger Dryas/Preboreal boundary is particularly abrupt with a sharp temperature rise of 5-7°C. The Last Glacial Maximum between 24,000 and 18,000 cal-yr-BP was a relatively mild period with SSTs on the order of 13-14°C. The abrupt coolings centered at about 17,000 and 25,000 cal-yr-BP are clearly concomitant with the Heinrich 1 and Heinrich 2 events. These climatic events were already detected in this area and probably represent the southeastern maximum extension of iceberg transport during Heinrich events (Baas et al. 1997). By combining the records of SST, 18O, global sea levels it is possible to evaluate in a semi-quantitative manner the local paleo-SSS (Duplessy et al. 1993). New estimates will be compared with another SSS proxy based on the percentage of the C37:4 alkenone (Rosell-Melé 1998). Indeed several cold minima in core SU8118 are characterized by increase of C37:4 by several percents.
Baas JH, Mienert J, Abrantes F, Prins MA, Paleo-3, 130, 1-23, (1997).
Bard E, Geochimica Cosmochimica Acta, 62, 2025-2038, (1998).
Bard E, Arnold M, Maurice P, Duprat J, Moyes J, Duplessy JC, Nature, 328, 791-794, (1987).
Duplessy JC, Bard E, Labeyrie L, Duprat J, Moyes J, Paleoceanography, 8, 341-350, (1993).
Rosell-Melé A, Paleoceanography, in press, (1998).
Multi-proxy SST profiles have been generated for core M35003 from the western subtropical Atlantic. The core is near the gateway for warm surface-water throughflow into the Caribbean. Age control is derived from 18O stratigraphy and 14C-AMS dating. Sedimentation rates are high at the core site, time resolution of the records varies between 300-800 yrs.
SST estimates are derived from a suite of independent temperature proxies: planktonic foraminiferal census counts, using multivariate statistics (Transfer Function Technique, TFT; Imbrie & Kipp, 1971), Modern Analogue Technique (Mat; Prell 1985) and Revised Analogue Method (Ram; Waelbroeck, 1998); long-chain alkenone (Rühlemann et al 1998); Mg/Ca ratios in G. sacculifer trilobus using a modified Mg/Ca vs. SST calibration (Nürnberg & Müller, 1998).
The records show rapid SST fluctuations during the last 50 kyrs similar to millennial-scale variability seen in paleoceanographic records at other sites in the tropical-subtropical and high-latitude Atlantic. Differences exist between the different SST records both in absolute SST and amplitudes of SST change. Strongest cooling events are documented by the planktonic foraminiferal assemblages during stage 3 when short-term changes between tropical-subtropical assemblages (dominated by G. ruber (w) and G. sacculifer trilobus) and moderate climate assemblages (dominated by G. bulloides) occur which translate into SST decreases by 3°-4°C. Mg/Ca ratios also indicate cooling during these events, however estimated SST amplitudes are only 1°-1.5°C and do not raise above mean Mg/Ca-derived SST variability; mean stage 3 SST derived from Mg/Ca is 0.5°-1°C warmer than TFT-derived SST. During stage 2, alkenone-derived SSTs fall well within the range of SST derived from planktonic assemblages and Mg/Ca; during stage 1 they are at the warm end of the multi-proxy SST range. Likewise, summer SST derived from TFT is similar to or periodically colder than annual mean SST derived from RAM.
Varying nutrient supply, carbonate dissolution, and organic matter degradation are discussed as secondary effects on the SST proxies. Carbonate preservation (high vs. low magnesium calcite) changes and benthic 13C and Cd/Ca provide additional control on shifts in ambient bottom water chemistry and potential post-depositional alteration of the proxies. We conclude that SST trends can be reliably inferred from the records, whereas absolute SST levels and amplitudes of SST changes need to be viewed with caution.
Imbrie, J. and Kipp, N., The late Cenozoic Glacial Ages, ed. by Turekian, K.K., 71-181, (1971).
Prell, W.L., U.S. DOE REP. TR 025, 60 p., (1985).
Waelbroeck, C., et al., Paleoceanography, 13, 272-283, (1998).
Rühlemann, C., et al., in prep., (1998).
Nürnberg, D. and Müller, A., in prep., (1998).
There is considerable interest in the use of Mg/Ca in planktonic foraminiferal calcite to proxy sea surface temperature. Also, planktonic Sr/Ca has been used to explore glacial-interglacial changes in seawater chemistry which may be attributable to sea-level changes and /or changes in continental weathering rates. These proxies require proper calibration. To this end, we have measured Mg/Ca and Sr/Ca in about 20 different species of planktonic foraminifera in six different size fractions from the top of BOFS core 31K from 3300 m off NW Africa. We have also carried out electron probe investigations of internal chemical variability for these species. The results will be shown in our poster.
Our understanding of the thermal history of the Cenozoic and late Mesozoic oceans derives largely from the application of the now well-established method of 18O-palaeothermometry to microfossil calcite. However, poor constraints on the extent of microfossil preservation and the 18O of the host waters of calcification introduce significant uncertainties to reconstructions of ancient sea water temperatures. Therefore, it is important to develop additional proxies with the potential to yield palaeotemperature records. One such proxy that has received attention in modern and Quaternary sediments is Mg/Ca in microfossil calcite.
Here we present the results of an investigation into the potential of applying the Mg/Ca technique to the problem of palaeotemperatures in the pre-Quaternary oceans using shallowly buried deep sea sections with excellent age control. Specifically, we present new Mg/Ca and 18O records in benthic foraminiferal calcite from the lowermost upper Eocene sediments (planktonic foraminiferal zone P15) with a burial depth of only 50 m from Ocean Drilling Program Site 1052 (Blake Nose, palaeowater depth ~750 m). These are also compared to Mg/Ca and 18O records from Site 689 (Maud Rise, palaeowater depth ~1700 m) and Site 523 (Walvis Ridge, palaeowater depth ~3100 m). Our Mg/Ca data show consistent interspecies differences, as do the oxygen isotopes. Over the short interval examined, the Mg/Ca variability is small compared to 18O, but the two parameters appear to vary in a consistent manner (i.e. in opposite phase). These results, combined with textural and chemical evaluation of individual specimens by electron beam micro-analysis suggest that these benthic foraminifera preserve primary Mg/Ca values. Application of a recently published Mg/Ca-temperature calibration derived from a modern epifunal species, C. floridanus to our Site 1052 data from an Eocene species with a similar assumed palaeoecology, C. mexicanus, yields average intermediate water temperatures over Blake Nose of ~8.5 ±2.0°C (assuming modern sea water Mg/Ca). These temperatures compare favourably to the temperature range obtained using 18O (~8.0 to 9.0 ±1.0°C). The coherence of the two calibrations suggests that Eocene sea water Mg/Ca ratio was not markedly different from today's value of 5.16.
Six different species of prymnesiophytes isolated from different parts of the Baltic Sea have been cultured to identify the sources of sedimentary alkenones in this large brackish water body. Prymnesiophytes, a group of algae that are known to have alkenones in their cell membrane, occur in different populations in salinities ranging from 30 to 6 psu in the modern Baltic Sea. In cooperation with several biological institutions around the Baltic Sea, we were able to grow monocultures of Chrysochromulina polylepis, Chrysochromulina apheles, Pavlova lutheri, Prymnesium parvum, Pleurocrysis carterae and Prymnesium patelliferum. The algae were cultured under optimal growth conditions at 20°C and salinities corresponding to the ambient salinity of the area where the respective species had been isolated (6 -30 psu). Our results show differences in the ratio of biomass to alkenone production. Pavlova lutheri produced the largest amount of alkenones per dry weight (817 ng/g) and thus might be an important producer of alkenones in the Baltic Sea. The alkenone contents of the other species were lower: Chrysochromulina polylepis, 223 ng/g; Prymnesium patelliferum, 208 ng/g; Chrysochromulina apheles 100 ng/g. Only traces were found in cultures of Prymnesium parvum and Pleurocrysis carterae. To get first informations about a possible relationship between the Uk'37 and the growth temperature of the prymnesiophytes, Chysochromulina apheles was grown at 11°C, 14°C, 16°C, 18°C and 20°C at 30 psu. Our first results show an increasing Uk'37 with decreasing temperature from 0.509 at 20°C over 0.593 at 18°C and 0.659 at 16°C. However, more studies of temperature-dependent alkenone production in Baltic Sea prymnesiophytes are needed to affirm these first results.
Long-chain alkenones with 37-40 carbon atoms are synthesized by prymnesiophytes and are ubiquitous biomarkers in the water column and the sediments of the oceans. In open sea environments, the Uk´37 index covaries with sea-surface temperature. In addition, evidence from certain high-latitude marine environments suggests that the relative abundance of tri- and tetraunsaturated C37 ketones is related to the salinity of ambient water. While alkenones have also been reported from lacustrine environments, there is no experimental basis which would permit their use as paleotemperature indicator in lakes. Surface sediments of the Baltic Sea contain alkenones that show relative decreases in the C37:3/C37:4 ratio (from 19 to 2) which may reflect the modern salinity gradient from 30 to 5 psu or may correspond to different prymnesiophyte populations. However, screening of extant prymnesiophyte species adapted to a range of optimal growth salinity levels (30 to 6 psu) show no species-dependent difference in the C37:3/C37:4 ratios. This may imply that the sedimentary signal results from salinity stress on individual species, which respond to salinity decrease by producing more C37:4. When comparing the C37:3/C37:4 ratios of alkenones with salinity-dependent diatom distributions in a sediment core from the Gotland Deep (present-day salinity in the surface layer of 7 psu) dating back 7600 years, we see indication of a gradual freshening in the Central Baltic Sea in both variables.The main growth season of prymnesiophytes in the modern Baltic Sea is in summer months. Calculation of SST based on Uk´37 according to the open-ocean calibrations yields temperatures that increase from 5°C during the Holocene climatic optimum to between 10°C and 15°C for the youngest samples. While the youngest temperatures estimates correspond quite well with observed climatological temperatures in the surface water layer, those of the middle Holocene appear to be too low. Besides temperature, alkenone unsaturation ratios in Holocene sediments may thus also reflect the salinity history of the Baltic Sea. Detailed environmental reconstructions have to await the results of ongoing culture experiments (see companion abstract by Menzel et al.).
As part of the NAUSICAA-IMAGES II program, paleotemperatures will be appraised from the alkenone unsaturation index (UK'37) determined in sediment cores from the South-Western African margin. Bioeographical differences in the relationship between UK'37 and temperature might cause bias on SST estimates (Conte et al., 1992), it is therefore useful to validate established calibrations of UK'37 versus temperature for a given paleooceanographic study area. The present contribution explores the empirical relationship between in situ measured temperatures and UK'37 values determined in 21 surface particle samples recovered over the South Western African margin. Furthermore, the abundance and composition of alkenones is investigated with respect to the stocks and the diversity of coccolithophorid species.
In situ measured temperatures spanned over a range from 13 to 24°C. The UK'37 values were linearly related to temperatures (r2 = 0.85) and no flattening out of the curve was observed in the higher and lower temperature ranges. The slope of the linear regression line is higher than that of the top-core calibration recently reported by Müller et al. (1998) in the same area. It is also higher than the slope of the widely used calibrations of Brassell et al. (1986) and of Prahl et al. (1988). The regression compares well with that obtained from cultures of E. huxleyi and G. oceanica (Conte et al., 1998). The temperatures calculated using our empirical regression are generally within 1°C of the in situ measured temperature, whereas the Prahl et al. calibration yields greater differences. Unsaturation index of C38 Me alkenones is also linearly correlated with SST, and shows a slope similar to that obtained with UK'37 with however a greater scattering.
No significant relationship between total alkenones and total coccolithophorid cell is observed when the whole set of data is considered. Interestingly, alkenone concentrations are well correlated with coccolithophorid stocks for samples in which the relative abundance of E. huxleyi exceeds 85%. In samples with higher coccolithophorid diversity where E. huxleyi accounts for less than 85%, a significant correlation is also found between alkenone concentrations and coccolithophorid stocks. The two data sets yield markedly different slope suggesting species dependant cellular contents in alkenones. Nevertheless, such discrepancies might also reflect differences in physiological status rather than species composition of the coccolithophorid assemblage (Conte et al., 1998).
Conte MH, Eglinton G, Madureira LAS, Org. Geochem, 19, 287-298, (1992).
Brassell SC, Eglinton G, Marlowe IT, Pfaumann U, Sarnthein N, Nature, 320, 129-133, (1986).
Prahl FG, Muehlhausen LA, Lyle M, Geochim. Cosmochim. Acta, 52, 2303-2310, (1988).
Müller PJ, Kirst G, Ruhland G, Von Storch I, Rossell-Mele T, Geochim. Cosmochim. Acta, 62, 1752-1772, (1998).
Conte MH, Thompson A, Lesley D, Harris RP, Geochim. Cosmochim. Acta, 62, 51-68, (1998).
The Japan Sea is a semi-isolated marginal sea located between Japan and the Asian conti-nent, and is connected to the open north Pacific Ocean through shallow straits with sill depths of less than 130 m. Because of its geographical condition, the Japan Sea has been severely influenced by sea level changes in the Pleistocene/Holocene and became almost isolated from the open ocean during the last glacial. A goal of our research is to study changes in paleoceanographical conditions of the Japan Sea in galcial-deglaciation -Holocene by the analysis of alkenone-SSTs and to understand those changes in the context of climatic changes in the Northern Hemisphere.
In a previous paper , we reported variations in alkenone-SST at Oki Ridge in the Japan Sea over the last 17.5 ka (calendar age). Alkenone-SST fluctuates in the range of 14 -18°C in connection with sea level change: SST is high in 17.5 ka at the time of lowest sea level (Last Glacial Maximum), decreases from 18°C to 14°C in 17.5-11.6 ka, rises dramatically from 14°C to ~19°C in 11.6-11.1 and then becomes almost steady. However, our results of SST variations do not necessarily agree with those expected from microfossil analysis (e.g. the coiling ratio of the planktonic foraminifer Neogloboquadrina pachyderma).
Therefore, the present study was carried out to confirm the previous observation of variations in alkenone-SST and to discuss the reason of the discrepancy between alkenone-SST and the microfossil analysis. A sediment core KT94-15, PC-9 (water depth 807 m, core length 520 cm) was taken at 39°34.36'N, 139°26.28'E (by 450 km northeast of the previous coring site). The temporal variations in alkenone-SST from this core are very similar to those reported previously. In particular, we obtained relatively high alkenone-SSTs in the 30-17.5 ka. This high alkenone-SST is not consistent with those suggested from microfossil analysis. Several hypothesis can be proposed for explaining this discrepancy, which should raise points of further investigations.
Past sea surface temperatures (SST) and productivity in the southeastern subtropical Pacific Ocean have been reconstructed for the last 400 kyrs from the composition of the C37 alkenones. The study is based on a core situated in the Nazca Ridge (17o14'4''S, 78o6'16''W, 3120 m water depth) that exhibits uniform sedimentation rate (1 kyr/cm) and good preservation during the last 400 kyrs. The constructed age scale in based on foraminifera 18O isotope stratigraphy.
The SST profile follows the glacial / interglacial pattern for the last three climatic cycles. SST changes in all warmings to interglacial stages, including the last glacial maximum (LGM) to Holocene transition, are about 3-3.5oC. This temperature difference is consistent with those observed in the subtropical South Atlantic at the same latitude using the C37 alkenone method.
Lower temperatures than in LGM are observed in stages 2 and 3. The lowest temperature recorded, 16.4oC, is about 1.2oC lower than that in LGM.
Maximum SST in the interglacials prior to Holocene (stages 5e, 7 and 9) were 0.5-0.8oC higher than present. Lowest SST prior to deglaciation in stages 6 and 8 (but not in stage 10) were also about 0.5-0.8oC higher than in stage 2.
Algal productivity examined from the C37 alkenone fluxes shows a strong climatic dependence being inversely correlated with SST. The warmer periods are those exhibiting lower C37 alkenone abundances and viceversa. This trend is consistently observed throughout the three climatic cycles examined.
Since their first description, alkenones have been found in numerous marine sediments. Their application for estimating paleo-sea surface temperatures has been demonstrated in many different marine environments and is not restricted to sediments younger than ca. 268 ka, the first occurrence Emiliania huxleyi, nor to the occurrence of the previously dominant Geophyrocapsa oceanica. Other living and extinct members of the family Geophyrocapsaceae have or had the capability of synthesizing alkenones whose degree of unsaturation changes with growth temperature. Samples from the New Jersey continental slope (ODP Leg 150) clearly support the applicability of the alkenone proxy for sediments back to the Eocene. For example, sea-surface temperatures of more than 20°C in the Eocene, rapid cooling in the late Oligocene and warming in the early Miocene excellently fit to global sea-level-change reconstructions by Haq et al. (1987). Another temperature-sensitive proxy is the average chain length (ACL) of terrestrial n-alkanes. Poynter (1989) found a correlation between pollen record and ACL values for West African continental margin sediments (ODP Holes 658A and 658B) during the last 24 ka. A coupling with alkenone temperatures is obvious at the New Jersey continental rise and slope back to the Miocene (ODP Leg 150), for late Quaternary sediments from the California continental margin (ODP Site 893, Santa Barbara Basin, and ODP Leg 167) and for Pliocene/Pleistocene Mediterranean sapropels (ODP Leg 160).
Haq BU, Hardenbol J & Vail PR, Science, 235, 1156-1166, (1987).
Poynter JG, Ph. D. dissertation, (1989).
This study is based primarily on analyses of core samples from several exploration and production wells along the Norwegian continental margin. Material from sidewall cores and ditch cutting samples were also used in some wells. Core samples from ODP Site 986, on Svalbard margin, were also included. The analysed boreholes cover a transect from the Ekofisk Field in the central North Sea to the Svalbard margin. The dataset gives new information for the understanding of the paleo environment in northern areas and on the Norwegian shelf in Tertiary time. Oxygen isotopic compositions of the tests of planktonic and benthonic foraminifera gives a temperature estimate for Paleogene and Neogene. Deep water sediments from Neogene, and especially Paleogene, is charaterized by carbonate barren intervals because of the shallow CCD level in the Norwegian Sea during this period. It has therefore been necessary to use continental shelf and margin material to reconstruct the thermal history. Chronologic control rest upon established biostratigraphy by the Norwegian Petroleum Directorate and 87Sr/86Sr analyses on foraminifera tests and bivalve fragments. The numeric ages derived from Howarth and McArthur (1997) look up table agree well with the biostratigraphy that exist for these wells and Sr-isotope stratigraphy can give better age determination than biostratigraphy for certain periods in Neogene and Paleogene.
Howarth RJ & McArthur JM, Journal of Geology, 105, 441-456, (1997).
The Neogene is a time of fundamental climatic and paleoenvironmental change. Thus it presents a series of examples for climatic situations differing from the present. These examples include environments with increased contents of greenhouse-gases as well as environments with cooler conditions than at the present. Within the tasks of UNESCO Project IGCP-341 paleotemperature transfer-algorithms (not functions) have been developed that permit the assessment of paleo temperatures from faunal and floral communities well back into the Miocene. These methods have been applied to about 600 drillholes from the Deep Sea Drilling Project /Ocean Drilling Program enabling also the "recycling" of a significant number of older wells.
Ocean currents, changed intensities of ocean currents and impacts of orographic changes such as the closing of the Isthmus of Panama show up consistently. As each sample is processed only using the fauna observed (the system does not receive any regional or stratigraphic information for paleo temperature assessment) the cosnsistency of the results as well as the control-runs show the reliability of the method.
Time series of paleo temperatures range well back into Miocene.
The derived maps permit the coupling of the data with a large scale atmospheric circulation model. Doing this parameters that are difficult to reconstruct could be approximated as well.
Thus IGCP-341 (reconstructing, mapping and modeling) is provides for the warmer times what CLIMAP did for 18K.
The primary and, in fact, only source of information on palaeoclimates are so called proxies - remnants of organic or non-organic origin, which by various physical, chemical or biological mechanisms have been influenced by past climates. For a number of reasons, the quality of proxies varies strongly and generally cannot be considered satisfactory for palaeoclimate reconstruction purposes. Moreover, also spatial and temporal density of accessible proxy-data is very heterogenous and globally poor. As a result, only very fragmentary and often inconsistent picture of past climate can be revealed from proxies by geological and palaeontological methods alone. Here an application of a promising method (first proposed by Mosbrugger and Schilling, 1992) is presented, which potentially allows a critical scrutiny of all accessible and climatically relevant proxy-data from a time slice. This is performed by means of numerical modelling of the atmosphere on both global and regional scales. The results of the simulations depend onthe boundary conditions, which only can be supported by proxies. A numerically produced, internally consistent, model-data frame facilitates an identification of inconsistencies among heterogeneous proxy-data. Moreover, it produces a physically meaningful extrapolation of those accepted proxy-data, avoiding arbitrariness of purely mathematical extrapolation routines. The method was applied to proxy-data aged ca. 8 Ma (Upper Miocene, Tortonian). Atmospheric simulations were carried out globally and for our chosen, proxy-data-rich region of the Lower Rhine Embayment in Germany. We could detect and eliminate some inconsistencies and contradictions in palaeobotanical proxy-data. A global picture of Tortonian climate, as well as a focused view at the region fit well the knowledge on the boundary conditions of the time. These are globally characterised by an orography substantially different from present, and by Northern polar region free from permanent ice. A weakening of the Gulf Stream, the reduced role of the westerly winds in Middle Europe and strenghtening of small scale climatic structures combine together with the boundary conditions to produce a consistent global/regional picture.
The oxygen isotopes record of planktonic foraminifera from the tropical core MD77194 (Eastern Arabian Sea at 10N, 75E) exhibits a clear two steps deglaciation (Duplessy et al. 1981). These 18O fluctuations can be correlated with those from core 74KL which is accurately dated by AMS of 14C (Sirocko et al. 1993). The first decrease in 18O is in phase with the Bölling initial warming while the second deglaciation step occurs after a brief 18O transient concomitant with the Younger Dryas (YD) event. Paleo- Sea Surface Temperatures (SSTs) were reconstructed with alkenones (UK37') (Sonzogni et al. 1998) and from planktonic foraminifera using the Modern Analog Technique (MAT). The alkenone-SST record shows a two step character in phase with the 18O record: a 1.5°C warming at about 15 cal-kyr-BP and a second warming of about 1°C starting at about 12 cal-kyr BP. A small temperature decrease occurs between 13 and 12 cal-kyr-BP in phase with the transient 18O increase. The MAT-SST record shows a 3°C warming leading by a couple of kyr the first deglacial 18O decrease. Interestingly, the cold season MAT-SST exhibits a 1°C cooling during the YD event. The annual Primary Production (PP), estimated from planktonic foraminifera using a transfer function (Cayre et al. in press), remains at about 450 gC.m-2.yr-1 during the last glacial period and near about 150 gC.m-2.yr-1 during the Holocene. These PP variations agree well with the qualitative productivity estimations based on the total abundance of C37 alkenones. The 3°C warming reconstructed wih the MAT corresponds to the drastic decrease in annual PP but the 1°C cooling of the cold-SST during YD is not correlated with an annual PP increase. Consequently, these latter variations of the cold-SST are not a methodological artefact. Indeed, samples from this interval are characterized by real changes of the foraminiferal assemblage with a relative abundance of Globigerinoides ruber higher than 40%. Consequently, both alkenone- and MAT-SST records suggest that the transient increase of 18O during the YD corresponds to a decrease in the temperature of calcification rather than to a change in the isotopic composition of the surface water. This contrasts with the conclusion by Anderson & Thunell (1993) that YD 18O events at low latitudes are always due to redistribution of oxygen isotopes through the water cycle.
Anderson DM, Thunell RC, Quat. Sci. Rev., 12, 465-473, (1993).
Cayre O, Beaufort L, Vincent E., Quat. Sci. Rev., in press
Duplessy JC, Bé AWH, Blanc PL, Paleo-3, 33, 9-46, (1981).
Sirocko F et al., Nature, 364, 322-324, (1993).
Sonzogni C, Bard E, Rostek F, Quat. Sci. Rev., 17 :12, 1185-1201, (1998).
Alkenone unsaturation indices (Uk'37) give precise estimates of past sea-surface temperatures (SST), and sedimentary alkenone concentrations provide a partial proxy of marine primary productivity. We present a high-resolution record of alkenone data from ODP Site 1002 in the Cariaco Basin. Our record spans the last 160 kyr with a mean sampling interval of 600 yr. Despite the low latitude (11°N) of the study site, we infer a wide range (6°C) of SST variability from our Uk'37 data; however, the complex pattern of variability differs considerably from that seen in a corresponding planktonic 18O record. Although warmest SST occurred during the previous interglacial period (Marine Isotope Stage 5e), coldest SST did not occur during either of the last two glacial maxima. Alkenone concentrations also vary over a wide range (0.01-13 µg g-1) and in a pattern generally similar to organic C, with lowest concentrations during glacial maxima and high concentrations during interglacial periods, but several intervals of greatest alkenone abundance do not contain proportionally high amounts of organic C.
Dissimilarities between the Uk'37 and 18O records suggest that the isotope signal includes a strong local salinity component, whereas changes in the relative proportions of alkenones and organic C could reflect shifts in the ecological balance of primary producers. Extreme low stands of sea level during glacial maxima could strongly affect the salinity and nutrient supply within the physically isolated Cariaco Basin and thus could enrich its isotopic composition and greatly inhibit its productivity without a large change in SST. Moreover, a survey of other cores indicates that large spatial variations in productivity have occurred within the basin during the Holocene. High-amplitude changes in SST and alkenone concentration that characterize the period from 70-110 ka otherwise remain difficult to explain.
The study of the UK37 index in four sediment cores from the South China Sea (SCS) (17940, 17954, 17961 and 17964) obtained during R/V SONNE cruise in April-June 1994, has allowed the reconstruction of past sea surface temperatures (SST) during the last 220 kyrs. The cores are situated in the northern and southern SCS and exhibit SST profiles that follow the glacial / interglacial pattern with a north-south difference of 1oC in the interglacials, and nearly 2.5oC in most of the glacial stages (3oC in stage 6). These differences define a temperature offset that remains constant even in episodic heating events such as the SST oscillation centered at 85 ka BP.
These north-south temperature differences define two distinct climatic regimes for the SCS that roughly correspond to glacial / interglacial periods, with high SST difference for the glacials and low difference for the interglacials. Transition from one pattern to the other seems to have occurred over short time periods such as at the end of stage 5e and during warming from LGM to Holocene conditions. These changes probably correspond with sea level fluctuations that permitted or prevented surface sea water exchange with the tropical Indo-Pacific waters in the south.
In both in the northern and southern SCS, the UK37-SST differences between LGM and Holocene are between 1 - 3oC higher than those observed at the same latitudes in the Atlantic and Pacific Oceans. This temperature contrast is in agreement with other SST estimates based on foraminiferal assemblages and points to an amplification mechanism that increased the glacial / interglacial differences in this marginal sea.
The deglacial 18O and SST profiles exhibit well defined Bølling-Allerød and Younger Dryas events in Termination I, particularly in the north. The Younger Dryas event show a cooling of 0.4 - 0.8oC with respect to the Bølling-Allerød temperatures in the south and north, respectively.
Comparison of Holocene and stage 5e SST shows warmer temperatures (1 - 1.2oC) in the previous interglacial period. SST were remarkably stable during stage 5e, which contrasts with the significant fluctuations in the 18O record. The highest temperature observed in stage 7 lies between that of the Holocene and stage 5e.
Time-series of UK'37 values are increasingly used to reconstruct the thermal history of Quaternary oceans. The UK'37 proxy is calculated from relative concentrations of two lipids synthesised by specific species of haptophyte algae and subsequently incorporated into the underlying sediments (Brassell et al., 1986). The linear relationship between UK'37 values in surface sediments and modern sea surface temperatures (SST) has been thoroughly investigated to produce calibration equations such as the recent one for the global ocean (Muller at al., 1998). The potential effects of sedimentary organic matter (OM) diagenesis on Uk'37 have caused concern over its applicability in certain environments. Conclusions drawn from investigations of sediments experiencing high levels of OM oxidation have been contradictory. Prahl et al. (1989) concluded that highly oxidising environments do not significantly alter Uk'37 whereas Hoefs et al. (1998) observed increased degradation of the more unsaturated lipid and concluded that application of Uk'37 to derive absolute SSTs may not be justified. Both studies were conducted on samples from the Madeira Abyssal Plain taken across a steep redox gradient in turbidite sequences. However, the samples used by Hoefs et al. were between 20 and 100 times older than the Pleistocene samples used by Prahl et al.
The preservation of the proxy signal over m.y. time-scales requires investigation before application of Uk'37 to late Neogene climate reconstruction. The results presented here are from a pilot study investigating the use of Uk'37 in pre-Quaternary samples. Three Uk'37 derived SST time-series at ~50 k.y. resolution covering the past 5 Ma have been reconstructed for sites in the south-east South Atlantic, the eastern equatorial Pacific and the north-west Pacific oceans. Each site has experienced contrasting environmental histories affecting SST, productivity and sedimentary OM preservation and has a well-constrained age model with suites of complementary palaeoceanographic proxy data.
The rates of SST change for the South Atlantic site indicate progressive cooling from 4.5 to 0.7 Ma followed by a steady mean SST with high variability typical of Pleistocene glacial cycles. Similar patterns could be caused by the preferential degradation through time of the unsaturated Uk'37 lipid in this highly reducing environment. In addition, Uk'37 values for before 0.07 Ma pre-date the organisms that produce the dominant signal used in most modern core-top calibrations. However, the Pacific sites show similar patterns, albeit with differing magnitude, and other proxy data for this period reveal a similar trend of gradual cooling punctuated by rapid changes including Antarctic ice volume expansion at ~3.4 Ma and the onset of northern hemisphere glaciation at ~2.5 Ma (Frakes et al., 1992). The reconstructions presented here suggest that long-term Uk'37 stratigraphy represents the climate signal rather than a diagenetic signal. However, further work is required before absolute SST derivatives can be used with confidence for pre-Quaternary high-resolution reconstructions.
Brassell SC, Eglinton G, Marlowe IT, Pflaumann U & Sarnthein M, Nature, 320, 129-133, (1986).
Frakes LA, Francis JE & Syktus JI, Climate Modes of the Phanerozoic, Cambridge University Press, 274, (1992).
Hoefs MJL, Versteegh GJM, Rijpstra WIC, deLeeuw JW & Damste JSS, Paleoceanography, 13, 42-49, (1998).
Muller PJ, Kirst G, Ruhland G, vonStorch I & Rosell-Mele A, Geochimica et Cosmochimica Acta, 62, 1757-1772, (1998).
Prahl FG, de Lange GJ, Lyle M & Sparrow MA, Nature, 341, 434-437, (1989).
Reconstructing sea surface temperatures (SSTs) is a crucial, but unfortunately most difficult task of paleo-climatology. In particular for the pre-Quaternary, SST proxy-data, such as 18O data, are poor in quantity and quality. Here we use a coupled atmosphere--mixed-layer-ocean model to derive the global sea surface temperature distribution for the Upper Miocene (Tortonian, approx. 8 Ma BP) from few and sparsely distributed 18O data; thereby the decoupled SSTs are calculated by using the flux corrections as a boundary condition.
In the Tortonian, mountain systems were considerably lower than today; the Himalaya, for instance, attained only about 50% of its present height. Furthermore, the southern limit of the ice margin of the North Polar Sea was located further North as indicated by proxy-data. Accordingly, Greenland was ice-free most of the year and had a different orography due to iso-static effects. 18O data suggest that the latitudinal SST gradient was smaller than it is today but the data are too scarce to run an atmospheric global circulation model for further analysis of the Tortonian climate.
To obtain paleo-flux-corrections for a coupled atmosphere mixed-layer-ocean model a method based on a comparison of gradients of paleo SSTs with those of recent SSTs is used. At first the following zonal averages are calculated: (1) zonal averages of the paleo SSTs at the few available 18O sites, (2) zonal averages of the recent SSTs at the same locations and (3) zonal averages of the recent SSTs over all latitudes and using all available recent data. For these three categories of data 'best-fit' Gauss curves over the latitudes are determined separately; thereby the Gauss curves (1) and (2), in contrast to Gauss curve (3), are only poorly supported by data. Subsequently the parameters of the recent Gauss curves (2) and (3) are related to each other to generate a transformation for these parameters. Applying this transformation to the paleo Gauss curve (1) one receives a more detailed and realistic paleo Gauss curve (4) over the latitudes. The ratio of the gradients of the paleo Gauss curve (4) over the recent Gauss curve (3) is then used to calculate the paleo flux-corrections from the recent ones.
With these paleo flux-corrections a model run with ECHAM 4 coupled to a slab ocean was performed for the Tortonian. After attaining equilibrium, the model Tortonian-SST-data differ from the model SSTs of the present-day situation. The most noticeable feature is the northward shift of the meteorological equator. The ocean currents on the Northern hemisphere, such as the Gulf stream and the Kuroshio, are weaker and the SSTs are higher at high latitudes.
The method presented here may prove useful to run atmospheric models for different paleo-climatic situations, it is, however, not adequate for paleo-oceanographic studies.
1. Recently a number of monographs (over 10) and articles was published, which dealt with the geology and detailed Paleogene and Neogene stratigraphy of the North Pacific and framing structures. Problems of paleogeography and paleoclimate received special attention.
2. It was found out that several noticeable warrning events (three Paleogene and three Neogene events) and less significant climatic fluctuations occurred for the last 65 m.y. against the background of the general cooling. The events are reliably marked by the appearances of warm-water planktonic and benthic assemblages and paratropic floral elements in northern sections. Assemblages of planktonic foraminifers and mollusks of Paleocene and Bocene ages were recorded in the Kamchatka and Bering Sea regions (The Koryak Upland) and remains of palms and other thermophyllic plants were discovered in northern Kamchatka. The Neogene warming events are also well marked by faunas (mollusks and others) and floras (diatoms, spores and pollen). The climatic fluctuations can be also reflected in specific lithological features of Cenozoic deposits: abundant glendonites-gennoishes and ice-rafied boulders in Oligocene sediments, ice-rafied pebbles in some upper Miocene and Pliocene beds, and soon.
3. The data obtained shows that in general the warming events were shorter than the succeeding relative cooling events (for example, for the Neogene time the ratio of their duration was 1:3). In some cases we can distinguish warming events of 0.5 m.y. and less duration. Long- and short-term warming events are reflected first of all in spore-pollen spectra (higher percentage of Fagus and oth.) and the appearance of exotic forms of biota (Morozovella, Dos in ja, and 0th.).
4. The paleoclimatic fluctuations resulted in different-scale migrations of fauna and flora from the south to the north (during the warmings) and back (during the coolings). Sometimes, they migrated over 2000-3000 km, as evidenced by the successive substitution of warm- and cool-water biotic associations in sections at different paleolatitudes and in different provinces (from Japan and California in the south to Kamchatka and Alaska in the north). We can suggest the effect of peculiar climatic "machine" in the North Pacific, which moved the northern subtropic boundary alternatively in the northern and southern directions. Naturally, this affected the distribution of biotic associations in the sea basins and floras in the continents. We have compiled some maps of the migrations for many Paleogene and Neogene time intervals.
5. Although the correlations outlined between the paleoclimatic fluctuations and eustatic sea level oscillations seem well substantiated in general, they need to be carefully studied and used with caution.
Since more than two decades, reconstructing paleo-temperatures from information recovered from ice cores, such as those from Antarctica and Greenland, is closely linked to the interpretation of the isotopic composition of the ice itself. Based on empirical and theoretical findings it is generally accepted that precipitation which is isotopically (H218O/H216O) enriched indicates relatively warm conditions during condensation whereas strongly depleted precipitation indicates cool conditions. This simple relation can be found for middle and high latitudes analyzing the measurements of a global network for isotopes in precipitation, GNIP. However, for using the isotopic composition of the ice as a paleo-thermometer a quantitative calibration is needed. Classically the regional slope between the annual mean surface temperatures and the corresponding isotopic composition of surface snow was used as an estimate for the paleo-thermometer. This classical interpretation has recently been challenged by independent studies on bore hole temperatures or on the thermal diffusion of various gaseous components of air enclosed in the ice. Using these methods bear some mathematical difficulties since the diffusion equations which are employed for both problems have no unique solution. However, for Greenland, these studies point to an overestimation of the classical temporal isotope/temperature relation by about 100%. Conversely, this would mean that the last glacial maximum on Greenland was twice as cold as has been estimated before. In this contribution an overview shall be given on the various methods and recent evolutions in the paleo-thermometry of ice sheets.
The extent of amino acid racemization and epimerization (AAR) is governed by time-temperature dependent chemical reactions: where the thermal history is known AAR should give dates; where independent dates are available it should provide evidence of thermal (climatic) changes. The useful range of the reaction extends back into the mid Pleistocene. AAR has been applied to a range of terrestrial materials including wood, bones, teeth, eggshells and molluscs. The chemical techniques are relatively straightforward the technique can be applied cheaply and rapidly. In spite of these potential benefits, AAR is a controversial tool and has only rarely been used to estimate changes in palaeotemperature. A number of outstanding problems still remain 30 years after the method was proposed, it is improbable that it can be widely used without first providing (i) adequate kinetic models, (ii) internal validation. In addition an individual value represents a combined integrated signal from the time of deposition to the present day. An example of using AAR will be given as an illustration of the scope and limitations of the method. The problem with the method is that the extent of AAR is an integrated value reflecting the combined time/temperature history from the time of deposition to the present day. The method is better at validating a time temperature framework in association with other method than providing a unique solution.
Conventional palynology has provided some of the most comprehensive terrestrial palaeoenvironmental information. The main advantages of pollen records over other proxy terrestrial environmental indicators are that they are available from various environments and cover long (geological) timescales. As such, they can eb used to study wide spatial and temporal variations.
Recent advances in stable isotope mass spectrometry and pollen processing techniques have allowed the measurement of the stable isotope composition of pollen exine. Using a novel processing technique we show how problems of carbon contamination, which existed with conventional acetylation techniques, and post-depositional diagenesis of pollen grains may be eliminated.
We present the first attempts to assess, quantitatively the relationship between the 13Cpollen and temperature using pollen from multiple tree species across Europe. Our results demonstrate a highly significant correlation between the 13C composition of pollen and the temperature experienced during the period of its formation. This temperature:isotope relationship may be extended back through the length of existing pollen records. Such information has immense potential for significantly improving our understanding of vegetation dynamics and linkeages between marine and terrestrial environments.
The techniques so far most widely used in pollen-based quantitative palaeoclimate reconstructions for the Quaternary have yielded a wealth of palaeoclimatic data. They have, however, important limitations as not all Pleistocene climate scenarios and plant communities are represented today. This lack of modern analogues may result in inaccurate or even unrealistic climate reconstructions. At the same time these methods are sensitive to taphonomic effects because they are based on relative pollen abundances, and they are not applicable to megafloral remains, which often can be identified to a lower taxonomic level than pollen and thus may yield more specific climate information.
Consequently there is a need for a method which is relatively robust to non-analogue-scenarios and taphonomic bias, i.e. is applicable to older periods of the Pleistocene and to assemblages from different depositional environments. We therefore analyzed the potential of the "indicator species approach" of Iversen (1944) and further developed its methodology. The principle of this approach is that the specific requirements with respect to at least two climate parameters are established for up to three taxa from an assemblage which are considered to be climatic indicator species. Then the climatic space in which these taxa can coexist is determined, resulting in a climatic reconstruction for the time and space of the assemblage's deposition.
In contrast to the original concept we evaluated all members from a given assemblage. Thus we established a databank containing taxa requirements with regard to six climate parameters as well as a computer algorithm executing the calculation steps. So far information for 85 taxa from European pollen records is available. To test the climatic resolution of the approach as well as the quality of our databank, we climatically evaluated modern micro- and megafloras; the resolution obtained reaches up to 2,5°C for January and July mean temperatures. Furthermore we used more than 2000 modern floras to statistically assess the most probable positions of the "real" climate value within the climatic spaces reconstructed from the calculations. By this procedure the climatic resolution of the method can be further increased. We applied this approach to Tiglian and Holsteinian pollen spectra from Lieth (northern Germany) and Lac du Bourget (French Alps). In both records cold, intermediate and temperate stages are clearly identifiable with our approach, with the resolution being highest in temperate stages. During the Nordende Temperate Stage of the Lieth section, the mean July temperature ranged between 17,5 and 19°C; thus this time interval was warmer than today. In contrast, at least parts of the Mid-Holsteinian from the Lac du Bourget section were probably cooler than today, as is implied from the pollen spectra yielding temperatures between 16 and 18°C.
Iversen J, Geologiska Föreningen Förhandlingar Stockholm, 66, 463-483, (1944).
SW Africa comprises various environments. From south to north (latitudes 34°S-13°S), Fynbos vegetation, desert/semi-desert, savanna, dry forest/woodland and Afromontane forest cover the subcontinent in which mean annual temperature (MAT) increases northward from 16°C in the Cape region to 24°C on the Angolan-Namibian highland. Palynological investigation on marine core GeoB1023-5 (17°09'S, 11°01'E) provides a continuous record with high temporal resolution showing significant vegetation changes in SW Africa[ref.1], which reflect climate change. Site GeoB1023-5 receives pollen derived from SW Africa between 13° and 21°S. In this region, the present MAT is 18-20°C in the Northern Namib desert, 22-24°C on the Angolan-Namibian highland and 20°-22°C in the NW Kalahari. During the LGM (21-17.5 ka BP, high abundance of pollen from Restionaceae and Ericaceae (Fynbos elements), and Asteroideae (temperate semi-desert elements) indicates that these plants migrated to the NW Kalahari, which suggest the MAT lowering to 16-18°C there. The earliest warming signals associated with the Last Deglaciation are indicated by the decline of Restionaceae at ca. 19.3 ka BP and decline of Asteroideae during 17.5-16.5 ka BP. The maximum of the warming is characterized by a maximum in savanna during 16-15 ka BP. For this period, the inferred vegetation changes suggest that the MAT increased to 19-20°C in the northern Kalahari. A severe arid and most probably also cold period followed during 14.4-12.5 ka BP. This period correlates with the Antarctic Cold Reversal in the Antarctic and enhanced upwelling in the ES Atlantic ocean indicated by the record of dinoflagellate cysts from the studied core. During 12.5-10.9 ka BP, warm and humid conditions are indicated by expansions of savanna, dry forest/woodland and Alchornea. During the Holocene, a steady increase of dry forest/woodland occurred during 9.3-6.3 ka BP. Substantial expansions of dry forest/woodland during 6.3-4.8 ka BP suggest the warmest conditions in the Holocene and the MAT in the NW Kalahari was 1-2°C higher than at present. After this climatic optimum, cooling is indicated by expansion of Afromontane forest and Miombo woodland to lower altitude during 4.8-3.3 ka BP. Expansion of dry forest/woodland during 3.3-2.5 ka BP suggests that temperature already increased near to the present level. During the last 2-3 millennia, the vegetation changes have been affected by human activities.
Shi N, Dupont LM, Beug H-J & Schneider R, Veget. Hist. Achaeobot., 7, 127-140, (1998).
Bivalve shells are known to produce distinct consecutive growth increments. Using a high precision microsampler, we analysed time series of shell growth increments for oxygen, carbon, and strontium isotope composition. Sample resolution, in all cases, is better than one month of growth. The shells studied are well preserved aragonitic bivalves from a fresh - brackish water fauna in the late Middle Miocene (± 11 - 15 Ma) Pebas formation in Western Amazonia (Colombia and Peru). For comparison, we also analysed modern bivalves from the same region (genus Anadontites and Diplodon), of which environmental parameters during growth are recorded.
The 18/O composition of shells is traditionally used as a paleo-temperature proxy in the marine environment. In contrast, in present day Western Amazonia the 18/O composition of natural waters is mainly governed by the variation in rainfall intensity. Small seasonal T differences which should result in a few tenths of per mil variation in 18/O are completely overprinted by the distinct difference in 18/O composition of natural waters in the dry and wet season
Although reliable quantitative paleotemperature reconstructions can not be expected on the basis of 18/O data alone, this comparison between modern and fossil shells does improve paleoenvironmental and paleoclimatic reconstructions of this area in the Miocene. For example: Carbon isotopic data of the fossil bivalve shells agree best with a lacustrine setting, as opposed to a riverine setting as their living environment. Further, Sr isotope data tell us that these bivalves lived in fresh water with only occasional brackish water influences in the top of the studied sequence.
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