Lowell D. Stott

Evolution of early Cenozoic marine temperatures

The equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals: late Paleocene, early Eocene, early middle Eocene, late Eocene, and early Oligocene. Paleotemperatures are calculated using standard oxygen isotope/temperature equations with adjustments to account for (1) variations in sea water δ18O related to changes in global ice volume over time and (2) latitudinal gradients in surface water δ18O. These reconstructions indicate that sea-surface temperatures (SST) of the Southern Oceans in the early Eocene were as high as 15°C, whereas temperatures during the late Paleocene and early middle Eocene reached maximum levels of 10°–12°C. By the late Eocene and early Oligocene high latitude SST had declined to 6 and 4°C, respectively. For most of the early Paleogene, low latitude sub-tropical temperatures remained constant and well within the range of Holocene temperatures (24°ndash;25°C) but by the late Eocene and early Oligocene declined to values in the range of 18° to 22°C. The late Paleogene apparent decline in tropical temperatures, however, might be artificial because of dissolution of near-surface foraminifera tests which biased sediment assemblages toward deeper-dwelling foraminifera. Moreover, according to recent plate reconstructions, it appears that the majority of sites upon which the late Eocene and early Oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling. Global deepwater temperature on average paralleled southern ocean SST for most of the Paleogene. We speculate based on the overall timing and character of marine sea surface temperature variation during the Paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high-latitude warmth of the early Eocene.

Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch

In the present-day climate, surface water salinities are low in the western tropical Pacific Ocean and increase towards the eastern part of the basin. The salinity of surface waters in the tropical Pacific Ocean is thought to be controlled by a combination of atmospheric convection, precipitation, evaporation and ocean dynamics, and on interannual timescales significant variability is associated with the El Niño/Southern Oscillation cycles. However, little is known about the variability of the coupled ocean–atmosphere system on timescales of centuries to millennia. Here we combine oxygen isotope and Mg/Ca data from foraminifers retrieved from three sediment cores in the western tropical Pacific Ocean to reconstruct Holocene sea surface temperatures and salinities in the region. We find a decrease in sea surface temperatures of ∼0.5 °C over the past 10,000 yr, whereas sea surface salinities decreased by ∼1.5 practical salinity units. Our data imply either that the Pacific basin as a whole has become progressively less salty or that the present salinity gradient along the Equator has developed relatively recently.

Magnitude and timing of temperature change in the Indo-Pacific warm pool during deglaciation

Ocean–atmosphere interactions in the tropical Pacific region have a strong influence on global heat and water vapour transport and thus constitute an important component of the climate system. Changes in sea surface temperatures and convection in the tropical Indo-Pacific region are thought to be responsible for the interannual to decadal climate variability observed in extra-tropical regions, but the role of the tropics in climate changes on millennial and orbital timescales is less clear. Here we analyse oxygen isotopes and Mg/Ca ratios of foraminiferal shells from the Makassar strait in the heart of the Indo-Pacific warm pool, to obtain synchronous estimates of sea surface temperatures and ice volume. We find that sea surface temperatures increased by 3.5–4.0 °C during the last two glacial—interglacial transitions, synchronous with the global increase in atmospheric CO2 and Antarctic warming, but the temperature increase occurred 2,000–3,000 years before the Northern Hemisphere ice sheets melted. Our observations suggest that the tropical Pacific region plays an important role in driving glacial—interglacial cycles, possibly through a system similar to how El Niño/Southern Oscillation regulates the poleward flux of heat and water vapour.

Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton

Recycled Cretaceous and Cenozoic marine microfossils have been recovered from samples of the Pliocene Sinus Formation. Samples were collected from outcrops in the Reedy, Beardmore, and Ferrar glacier areas of the Transantarctic Mountains between lat 77° and 86°S. The glaciogene sediments contained diatoms, foraminifera, calcareous nannoplankton, silicoflagellates, radiolarians, sponge spicules, palynomorphs, and ostracodes of Late Cretaceous, Paleocene, Eocene, late Oligocene, late Miocene, and Pliocene age. This suggests the presence of open marine basins on the East Antarctic craton during late Mesozoic and Cenozoic time. The apparent absence of early Oligocene and early through middle and earliest late Miocene assemblages suggests either that marine regression exposed the basin floors or that ice filled the basins during these times. The high-elevation setting of Sirius Formation outcrops suggests one of two hypotheses for their origin: (1) They are in situ Pliocene glaciomarine deposits that were uplifted 1,750–2,500 m with the Transantarctic Mountains to their present elevation; (2) the Sirius Formation deposits are a mixture of derived sediments stripped from sub-ice intracratonic basins and subsequently redeposited by ice flowing up the inland slope of the Transantarctic Mountains. We favor the second hypothesis, with transport to sites sometime within the past 3 m.y.

Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of ±0.2°C and ±0.5°C, respectively. The analysis of foraminifera suggests an interlaboratory variance of about ±8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about ±2–3°C. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about ±1–2°C) and a bias (about 1°C) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility.

Interlaboratory comparison study of calibration standards for foraminiferal Mg/Ca thermometry

An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752-1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSDr%), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSDR%), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.

Climate and hydrographic variability in the Indo‐Pacific Warm Pool during the last millennium

Planktonic foraminiferal Mg/Ca and delta O-18 derived sea surface temperature and salinity records from the Makassar Strait, Indonesia, show a long-term cooling and freshening trend, as well as considerable centennial-scale variability during the last millennium. The warmest temperatures and highest salinities occurred during the Medieval Warm Period (MWP), while the coolest temperatures and lowest salinities occurred during the Little Ice Age (LIA). These changes in the western Pacific, along with observations from other high resolution records indicate a regionally coherent southern displacement of the Inter-tropical Convergence Zone during the LIA, with more arid conditions in the northern tropics and wetter conditions in the southern tropics.

The influence of diet on the δ13C of shell carbon in the pulmonate snail Helix aspersa

Abstract The influence of diet and atmospheric CO2 on the carbon isotope composition of shell aragonite and shell-bound organic carbon in the pulmonate snail Helix aspersa raised in the laboratory was investigated. Three separate groups of snails were raised on romaine lettuce (C3 plant, δ13C=−25.8‰), corn (C4 plant, δ13C=−10.5‰), and sour orange (12C-enriched C3 plant, δ13C=−39.1‰). The isotopic composition of body tissues closely tracked the isotopic composition of the snail diet as demonstrated previously. However, the isotopic composition of the acid insoluble organic matrix extracted from the aragonite shells does not track diet in all groups. In snails that were fed corn the isotopic composition of the organic matrix was more negative than the body by as much as 5‰ whereas the matrix was approximately 1‰ heavier than the body tissues in snails fed a diet of C3 plant material. These results indicate that isotopic composition of the organic matrix carbon cannot be used as an isotopic substrate for paleodietary reconstructions without first determining the source of the carbon and any associated fractionations. The isotopic composition of the shell aragonite is offset from the body tissues by 12.3‰ in each of the culture groups. This offset was not influenced by the consumption of carbonate and is not attributable to the diffusion of atmospheric CO2 into the hemolymph. The carbon isotopic composition of shell aragonite is best explained in terms of equilibrium fractionations associated with exchange between metabolic CO2 and HCO3 in the hemolymph and the fractionation associated with carbonate precipitation. These results differ from previous studies, based primarily on samples collected in the field, that have suggested atmospheric carbon dioxide contributes significantly to the shell δ13C. The culture results indicate that the δ13C of aragonite is a good recorder of the isotopic composition of the snail body tissue, and therefore a better recorder of diet than is the insoluble shell organic carbon. Because the systematic fractionation of carbon isotopes within the snail is temperature dependent, the δ13C of the shell could provide an independent technique for estimating paleotemperature changes.

An Abrupt Shift in the Indian Monsoon 4000 Years Ago

Climates, Landsca Geophysical Mon

The upper Paleocene-lower Eocene stratigraphic record and the Paleocene-Eocene boundary carbon isotope excursion: implications for geochronology

Abstract The late Paleocene carbon isotope excursion which was first identified at ODP Site 690, is currently regarded as the best means for an exact correlation at a specific point in space and time between deep sea and continental stratigraphies. Yet its position relative to biostratigraphic datums in several outcrop and deep sea sections is apparently inconsistent. Based on the integration of magnetostratigraphic, biostratigraphic (calcareous nannofossils and planktonic foraminifera) and carbon isotopic data from the North (DSDP Sites 549 and 550) and South (ODP Site 690) Atlantic Ocean, we show that in order to understand the temporal relationships and sequence of late Paleocene to early Eocene biotic and isotopic events, it is necessary to construct a composite reference section. We infer from this that in addition to the well documented ‘classic’ carbon isotope excursion which occurs in mid-Biozone NP9, a younger isotopic excursion, yet undocumented, occurs in lower Biozone NP10. The 55 Ma age estimate in Chron C24r (0.66) used as a calibration in the construction of the latest geomagnetic polarity time scale in DSDP Hole 550 lies at an unconformity (separating stratigraphic levels low in Zone NP9 from low in Zone NP10) with the result that the position of the NP9/NP10 zonal/chronal boundary is higher/younger in Chron 24r. Because the sections recovered from DSDP Sites 550 and 690 probably do not overlap over the upper NP9-lower NP10 zonal interval, and because the former section is unconformable at the NP9/NP10 zonal boundary, there is currently no means of establishing satisfactorily a numerical chronology for Chron C24r, which means that the age estimates for the upper Paleocene to lowermost Eocene biostratigraphic and isotopic events await documentation of bio-, magneto-, and isotopic stratigraphy in continuous stratigraphic section(s).