Palaeotemperature and ocean chemistry records for the Palaeogene from Mg/Ca and Sr/Ca in benthic foraminiferal calcite

The Earth’s climate is understood to have undergone dramatic changes throughout the Cenozoic. The long-term cooling from the mid-Cretaceous “greenhouse” to the “icehouse” of today is believed to result from a combination of factors which alter the amount and distribution of heat over the Earth’s surface. These forcing factors include changing levels of greenhouse gases, the Earth’s albedo, and ocean-atmosphere circulation patterns which are influenced by the above parameters in addition to the opening and closing of ocean gateways. An understanding of these processes hinges on the availability of quantitative records of past climate change. Therefore, many proxy records have been compiled in an attempt to document the climatic variations of the past 100 million years. Quantification of the thermal history of the Cenozoic oceans depends largely on records of the oxygen isotopic composition (δO) of foraminiferal calcite from deep-sea cores. Benthic foraminiferal δO records are considered to be more reliable indicators of global change than planktonic foraminifera as the deep ocean is not affected by seasonal, latitudinal or geographical variations in temperature or salinity to the same extent as surface waters. However, the interpretation of these records is complicated by the fact that foraminiferal calcite δO is controlled by not only the temperature but also the δO composition of the sea water, and hence global ice volume. The form of the composite benthic foraminiferal δO-record for the Cenozoic shows a general increase (trend towards colder temperatures and/or increased ice volume) with rapid (<1 m.y.) excursions (c. 1‰) towards heavier values superimposed on the long-term signal (Miller et al. 1987). The first of these excursions occurred in the earliest Oligocene, and has been termed “Oi-1” (Miller et al. 1991). The temperature versus ice-volume proportions of this excursion are yet to be resolved (Zachos et al. 1996). Mg/Ca in benthic foraminiferal calcite has been shown to have potential as a palaeotemperature proxy, and a temperature calibration has been determined for a modern species, C. floridanus (Rosenthal et al. 1997). This method has been used to estimate Quaternary deep-sea temperatures (Martin et al. 1998). Here we analysed Mg/Ca in O. umbonatus from Deep Sea Drilling Project (DSDP) Site 522 across the Eocene–Oligocene boundary. An average of six individuals from the >125 μm size fraction were analysed per sample. Foraminifera were crushed between glass plates before ultrasonication in water and methanol to remove clays, oxidation (boiled for 10 min in 30% H2O2 + 0.1M NaOH) and leaching in weak acid (0.001M HNO3) for 30 sec. The samples were then dissolved in quartzdistilled 0.075M HNO3 before analysis for Mg/Ca by simultaneous ICPAES. This method gives precisions of 0.3–0.5% rsd. Comparison of the high-resolution benthic foraminiferal Mg/Ca record with a benthic δO-record (Zachos et al. 1996) across “Oi-1” confirms that the majority of the isotope excursion can be attributed to a rapid build-up of continental ice. Many of the processes thought to affect the Earth’s climate also influence the chemistry of the oceans. Therefore, records of past changes in ocean chemistry are important aids in palaeoclimatology. Records of sea water Sr/Ca can be used as a weathering proxy in geochemical carbon models. Benthic foraminifera have higher preservation potential than planktonic foraminifera and their presence in deep sea cores with excellent age-control gives them significant potential as a proxy for Cenozoic sea water Sr/Ca. Previous work has documented a relationship of decreasing Sr/Ca with water depth in modern benthic foraminifera (Elderfield et al. 1996). The reasons for this are unknown, but are unlikely to be temperature-related as inorganic experiments show that the Sr partition coefficient into calcite is inversely related to temperature (Katz et al. 1972). Two Ocean Drilling Program (ODP) and one DSDP sites with good magnetostratigraphic age control, shallow burial depth and Eocene palaeo-water depths ranging from 750 to 3100 m were sampled for benthic foraminifera (Table 1). Samples were cleaned using the technique described above, before high-precision analysis for Sr/Ca by isotope dilution thermal ionisation mass spectrometry (2 S.E.<0.5%). A number of monospecific downcore records were compiled, including two species (N. truempyi and O. umbonatus) which were analysed in all three sites. Examination of individual foraminifera by scanning electron microscope and electron microprobe indicate that the foraminifera are wellpreserved. Further evidence to support this belief is shown in the downcore records which contain consistent interspecific offsets in Sr/Ca. The late Eocene (planktic foraminiferal Zone P15) depth transect shows a similar trend of decreasing benthic foraminiferal Sr/Ca with increasing water depth to that seen in core-top samples. Significant interspecies offsets exist, but the rate of change of Sr/Ca with water depth is similar for all species analysed. This implies that benthic foraminiferal Sr/Ca has potential as a proxy for sea water Sr/Ca throughout the Cenozoic.