David W. Lea

Global temperature change

Global surface temperature has increased ≈0.2°C per decade in the past 30 years, similar to the warming rate predicted in the 1980s in initial global climate model simulations with transient greenhouse gas changes. Warming is larger in the Western Equatorial Pacific than in the Eastern Equatorial Pacific over the past century, and we suggest that the increased West–East temperature gradient may have increased the likelihood of strong El Niños, such as those of 1983 and 1998. Comparison of measured sea surface temperatures in the Western Pacific with paleoclimate data suggests that this critical ocean region, and probably the planet as a whole, is approximately as warm now as at the Holocene maximum and within ≈1°C of the maximum temperature of the past million years. We conclude that global warming of more than ≈1°C, relative to 2000, will constitute “dangerous” climate change as judged from likely effects on sea level and extermination of species.

Reevaluation of the oxygen isotopic composition of planktonic foraminifera: Experimental results and revised paleotemperature equations

Cultured planktonic foraminifera, Orbulina universa (symbiotic) and Globigerina bulloides (nonsymbiotic), are used to reexamine temperature:δ18O relationships at 15°–25°C. Relationships for both species can be described by linear equations. Equations for O. universa grown under low light (LL) and high light (HL) share a slope of −4.80 (0.21‰ °C−1) with a HL-LL offset of −0.33‰ due to symbiont photosynthetic activity. The effect of [CO32−] on O. universa is −0.002‰ µmol−1 kg−1 and is insensitive to temperature. For G. bulloides, ontogenetic effects produce size-related trends in temperature:δ18O, whereby larger shells are enriched in 18O relative to smaller specimens. The O. universa temperature:δ18O relationships are more accurate than previously published equations for describing plankton tow data. Our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic Cibicidoides data below 10°C. Temperature:δ18O relationships for G. bulloides provide good agreement with field data for this species from the northeast Pacific.

Effect of seawater carbonate concentration on foraminiferal carbon and oxygen isotopes

Stable oxygen and carbon isotope measurements on biogenic calcite and aragonite have become standard tools for reconstructing past oceanographic and climatic change. In aquatic organisms, 18O/16O ratios in the shell carbonate are a function of the ratio in the sea water and the calcification temperature. In contrast, 13C/12C ratios are controlled by the ratio of dissolved inorganic carbon in sea water and physiological processes such as respiration and symbiont photosynthesis. These geochemical proxies have been used with analyses of foraminifera shells to reconstruct global ice volumes, surface and deep ocean temperatures,, ocean circulation changes and glacial–interglacial exchange between the terrestrial and oceanic carbon pools. Here, we report experimental measurements on living symbiotic and non-symbiotic plankton foraminifera (Orbulina universa and Globigerina bulloides respectively) showing that the 13C/12C and 18O/16O ratios of the calcite shells decrease with increasing seawater [CO32−]. Because glacial-period oceans had higher pH and [CO32−] than today, these new relationships confound the standard interpretation of glacial foraminiferal stable-isotope data. In particular, the hypothesis that the glacial–interglacial shift in the 13C/12C ratio was due to a transfer of terrestrial carbon into the ocean can be explained alternatively by an increase in ocean alkalinity. A carbonate-concentration effect could also help explain some of the extreme stable-isotope variations during the Proterozoic and Phanerozoic aeons.

Controls on magnesium and strontium uptake in planktonic foraminifera determined by live culturing

Because strontium and magnesium occur in seawater with nearly constant ratios to calcium, variations in Sr/Ca and Mg/Ca in modern foraminifer shells are due to the influence of environmental parameters on calcification. We have cultured two species of planktonic foraminifera, Globigerina bulloides and Orbulina universa, to establish the influence of temperature, pH, and salinity. Experimental results indicate that temperature is the primary control on shell Mg/Ca and that shell Mg/Ca increases exponentially by about 8 to 10% per °C. The exponential rise in shell Mg with temperature mirrors the results from inorganic precipitation experiments and suggests at least partial thermodynamic control on Mg incorporation. Both seawater pH and salinity are secondary influences on shell Mg/Ca: −6% per 0.1 pH unit increase and +4% per salinity unit increase. Shell Sr/Ca responds far more weakly to environmental parameters, and the small range observed in shell Sr/Ca relative to measurement precision of the ICP-MS method used here limits how well controls on shell Sr can be determined. Higher temperature, salinity, and pH all appear to increase shell Sr/Ca, most likely through the kinetic influence of calcification. Our culturing results demonstrate the potential of Mg/Ca in G. bulloides as a paleothermometer. The culturing results suggest that the standard error of Mg paleothermometry is ±1.1°C, but when the secondary effects of salinity and pH are considered the error increases to ±1.3°C.

Glacial–interglacial changes in Subantarctic sea surface temperature and δ18O-water using foraminiferal Mg

Laboratory culturing experiments with living Globigerina bulloides indicate that Mg/Ca is primarily a function of seawater temperature and suggest that Mg/Ca of fossil specimens is an effective paleotemperature proxy. Using culturing results and a core-top Neogloboquadrina pachyderma calibration, we have estimated glacial–interglacial changes in sea surface temperature (SST) using planktonic Mg/Ca records from core RC11-120 in the Subantarctic Indian Ocean (43°S, 80°E) and core E11-2 in the Subantarctic Pacific Ocean (56°S, 115°W). Our results suggest that glacial SST was about 4°C cooler in the Subantarctic Indian Ocean and 2.5°C cooler in the Subantarctic Pacific. Comparison of SST and planktonic δ18O records indicates that changes in SST lead changes in δ18O by on average 1–3 kyr. The glacial–interglacial temperature change indicated by the Subantarctic Mg/Ca records suggests that temperature accounts for 40–60% of the foraminiferal δ18O change. We have used the Mg/Ca-based SST estimates and δ18O determinations to generate site-specific seawater δ18O records, which suggest that seawater δ18O was on average 1‰ more positive during glacial episodes compared with interglacial episodes.

155,000 Years of West African Monsoon and Ocean Thermal Evolution

A detailed reconstruction of West African monsoon hydrology over the past 155,000 years suggests a close linkage to northern high-latitude climate oscillations. Ba/Ca ratio and oxygen isotope composition of planktonic foraminifera in a marine sediment core from the Gulf of Guinea, in the eastern equatorial Atlantic (EEA), reveal centennial-scale variations of riverine freshwater input that are synchronous with northern high-latitude stadials and interstadials of the penultimate interglacial and the last deglaciation. EEA Mg/Ca-based sea surface temperatures (SSTs) were decoupled from northern high-latitude millennial-scale fluctuation and primarily responded to changes in atmospheric greenhouse gases and low-latitude solar insolation. The onset of enhanced monsoon precipitation lags behind the changes in EEA SSTs by up to 7000 years during glacial-interglacial transitions. This study demonstrates that the stadial-interstadial and deglacial climate instability of the northern high latitudes exerts dominant control on the West African monsoon dynamics through an atmospheric linkage.

Experimental determination of stable isotope variability in Globigerina bulloides: implications for paleoceanographic reconstructions

We have quantified the environmental and physiological parameters responsible for stable isotopic disequilibrium in the non-symbiotic planktic foraminifera, Globigerina bulloides, via controlled experiments with living specimens. Individual test chambers secreted in the laboratory were amputated, pooled with other chambers from defined positions in the shell whorl and analyzed for their carbon and oxygen isotopic composition. When temperature, δ18Owater and δ13C of ΣCO2 are kept constant, the chamber δ13C and δ18O values increase 2.6 and 0.8%. respectively between the smallest chambers (chs. 1–9, shell size ≈180 μm) and final chamber (ch. 14, shell size ≈500 μm). Feeding experiments with prey of different δ13C values show that 8–15% of the chamber δ13C signal is due to the incorporation of metabolic CO2. The observed ontogenetic trend is responsible for the stable isotope size-dependency in this species and may be due to a fractionation mechanism involving the incorporation of metabolic CO2 during calcification. Temperature experiments show that shell δ18O varies as predicted by paleotemperature equations, but is offset from equilibrium. We present correction factors that should be applied to δ13C and δ18O data from well constrained size ranges to yield either oxygen isotope equilibrium or ambient δ13C of seawater ΣCO2. Our results suggest that for paleoceanographic applications, shells in the 270–320 μm size range are optimal for paleoenvironmental reconstructions

Links between salinity variation in the Caribbean and North Atlantic thermohaline circulation.

Variations in the strength of the North Atlantic Ocean thermohaline circulation have been linked to rapid climate changes during the last glacial cycle through oscillations in North Atlantic Deep Water formation and northward oceanic heat flux. The strength of the thermohaline circulation depends on the supply of warm, salty water to the North Atlantic, which, after losing heat to the atmosphere, produces the dense water masses that sink to great depths and circulate back south. Here we analyse two Caribbean Sea sediment cores, combining Mg/Ca palaeothermometry with measurements of oxygen isotopes in foraminiferal calcite in order to reconstruct tropical Atlantic surface salinity during the last glacial cycle. We find that Caribbean salinity oscillated between saltier conditions during the cold oxygen isotope stages 2, 4 and 6, and lower salinities during the warm stages 3 and 5, covarying with the strength of North Atlantic Deep Water formation. At the initiation of the Bølling/Allerød warm interval, Caribbean surface salinity decreased abruptly, suggesting that the advection of salty tropical waters into the North Atlantic amplified thermohaline circulation and contributed to high-latitude warming.

Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca

Abstract We have generated benthic foraminiferal Mg/Ca records from eastern tropical Atlantic core M16772 (3.9 km) and eastern tropical Pacific core TR163-31P (3.2 km) to assess the potential for using benthic Mg-paleothermometry to reconstruct Quaternary bottom water temperature histories. Variations in Mg/Ca records from both the Atlantic and Pacific show a significant correlation with climatic oscillations of the last 330 kyr. Shell Mg/Ca peaks during interglacial episodes, with marine isotope stage (MIS) 5e Mg/Ca nearly matching Holocene values in both cores. Lower Mg/Ca values occur during glacial intervals. To estimate temperatures from downcore Mg/Ca, we have augmented the published Cibicidoides sp. Mg/Ca–temperature dataset [Rosenthal et al., Geochim. Cosmochim. Acta 61 (1997) 3633–3643] with new data to include the temperature range of deep waters. Applying the 10.9% change in Mg/Ca per °C defined by the expanded calibration to the deep Atlantic data implies glacial–interglacial shifts in deep water temperature of 2–4°C over the last 300 kyr. Temperature estimates are comparable to deep Atlantic temperature changes proposed by Labeyrie et al. [Nature 327 (1987) 477–482]. There is greater uncertainty in deriving temperatures from the tropical Pacific core due to a limited Uvigerina spp. core top calibration set; however, glacial–interglacial temperature oscillations appear to be on the order of 2–3°C. In addition, core TR163-31P records clear millennial-scale Mg/Ca and δ 18 O oscillations in MIS 3 corresponding to temperature fluctuations of >0.5°C. Additional calibration studies are needed to address potential secondary effects on Mg/Ca.

Reassessing Foraminiferal Stable Isotope Geochemistry: Impact of the Oceanic Carbonate System (Experimental Results)

Laboratory experiments with living planktic foraminifers show that the δ13C and δ18O values of shell calcite decrease with increasing sea water pH and/or carbonate ion concentration. The effect has been quantified in symbiotic (Orbulina universa) and non-symbiotic (Globigerina bulloides) species and is independent of symbiont activity and temperature. It is concluded that a kinetic fractionation process affects both the carbon and oxygen isotopic composition of the shell simultaneously. At present it cannot be determined definitively whether the relationship is controlled by the pH dependent balance between hydration and hydroxylation of CO2 or by [CO3 2-] related variations in the calcification rate. However, independent of which factor ultimately controls the relationship between the carbonate chemistry and isotopic fractionation, in the real ocean [CO3 2-] and pH covary linearly across the relevant pH range. The true relationship between shell isotopic composition and the bulk carbonate chemistry is masked by the fact that host respiration and symbiont activity locally modify the carbonate system. Respiration lowers and photosynthesis increases ambient pH and [CO3 2-]. This translates into modified absolute shell values but leaves the slope between the shell isotopic composition and the bulk carbonate chemistry unaffected. A second level of shell isotopic modification is introduced by the incorporation of respired carbon, enriched in 12C, which depletes the shell δ13C value. In symbiont bearing species this depletion is partially negated by a shell δ13C enrichment in the light. As an alternative to the RUBISCO hypothesis (enrichment via preferential removal of 12CO2), we propose that scavenging of respired CO2 during photosynthesis, raises the shell δ13C value. Our results have partly been documented before (Spero et al. 1997) and demonstrate that the carbonate chemistry is undoubtedly a major control on temporal geochemical variability in the fossil record. For instance, the sea water carbonate system of the pre-Phanerozoic world (Berner 1994; Grotzinger and Kasting 1993) or during glacials (Sanyal et al. 1995) was significantly different from today confounding direct interpretation of foraminiferal stable isotope data using existing relationships (see companion paper in this volume by Lea et al.).