Stable isotope record, Mg/Ca ratios and age model of St. Stephens Quarry, Alabama

In the largest global cooling event of the Cenozoic Era, between 33.8 and 33.5 Myr ago, warm, high-CO2 conditions gave way to the variable 'icehouse' climates that prevail today. Despite intense study, the history of cooling versus ice-sheet growth and sea-level fall reconstructed from oxygen isotope values in marine sediments at the transition has not been resolved. Here, we analyse oxygen isotopes and Mg/Ca ratios of benthic foraminifera, and integrate the results with the stratigraphic record of sea-level change across the Eocene-Oligocene transition from a continental-shelf site at Saint Stephens Quarry, Alabama. Comparisons with deep-sea (Sites 522 (South Atlantic) and 1218 (Pacific)) d18O and Mg/Ca records enable us to reconstruct temperature, ice-volume and sea-level changes across the climate transition. Our records show that the transition occurred in at least three distinct steps, with an increasing influence of ice volume on the oxygen isotope record as the transition progressed. By the early Oligocene, ice sheets were ~25% larger than present. This growth was associated with a relative sea-level decrease of approximately 105 m, which equates to a 67 m eustatic fall.

(Table 1) Trace element/Ca ratios of Orbulina universa shells from late Neogene Mediterranean samples

A Mediterranean composite sedimentary record was analyzed for Ba/Ca ratios on carbonate shells of Orbulina universa planktonic foraminifer (Ba/Ca)carb providing the opportunity to study and assess the extent of freshwater inputs on the basin and possible impacts on its dynamics during the Tortonian to Recent period. A number of scanning electron microscope analyses and auxiliary trace element measurements (Mn, Sr, and Mg), obtained from the same samples, exclude important diagenetic effects on the studied biogenic carbonates and corroborate the reliability of (Ba/Ca)carb ratios in foraminifera calcite as indicators of seawater source components during the studied interval. A long-term trend with (Ba/Ca)carb values shifting from ~7 to 3 µmol/mol from the base of the Tortonian to the top of the Messinian is observed. The interval of the late Messinian salinity crisis, where biogenic carbonates are missing or strongly diagenized, represents a crucial passage not monitored in our record. At the base of the Pliocene, up to about 4.7 Ma, the (Ba/Ca)carb record shows a decreasing trend from ~4 µmol/mol stabilizing itself to an about constant value of 0.9 ± 0.3 µmol/mol for the whole Plio-Pleistocene interval. These results suggest a dramatic change in the continental runoff values, up to ~3-16 times higher during part of the late Neogene (Tortonian-early Pliocene), with a possible profound modification in the physical dynamics of the Mediterranean basin. First-order mass balance equations used to estimate barium and salinity budgets in the Mediterranean Sea during the late Miocene-early Pliocene interval support the hypothesis of an active connection of the basin with the Paratethys region and of a definitive restriction at the base of the Pliocene after about 0.7 Ma from the well-known Messinian Lagomare phase. They also open intriguing scenarios on possible circulation shifts during the Neogene.

Plio-Pleistocene Mg/Ca and TEX86 sea surface temperature records of ODP Sites 184-1143 and 165-999,

The western warm pools of the Atlantic and Pacific oceans are a critical source of heat and moisture for the tropical climate system. Over the past five million years, global mean temperatures have cooled by 3-4 °C. Yet, current reconstructions of sea surface temperatures indicate that temperature in the warm pools has remained stable during this time. This stability has been used to suggest that tropical sea-surface temperatures are controlled by some sort of thermostat-like regulation. Here we reconstruct sea surface temperatures in the South China Sea, Caribbean Sea and western equatorial Pacific Ocean for the past five million years, using a combination of the Mg/Ca, TEXH86-and Uk'37 surface temperature proxies. Our data indicate that during the period of Pliocene warmth from about 5 to 2.6 million years ago, the western Pacific and western Atlantic warm pools were about 2 °C warmer than today. We suggest that the apparent lack of warming seen in the previous reconstructions was an artefact of low seawater Mg/Ca ratios in the Pliocene oceans. Taking this bias into account, our data indicate that tropical sea surface temperatures did change in conjunction with global mean temperatures. We therefore conclude that the temperature of the warm pools of the equatorial oceans during the Pliocene was not limited by a thermostat-like mechanism.

Stable isotope and Mg/Ca record of Globigerinoides ruger morphotypes

Tests of the planktonic foraminifer Globigerinoides ruber (white; d'Orbigny) have become a standard tool for reconstructing past oceanic environments. Paleoceanographers often utilize the Mg/Ca ratios of the foraminiferal tests for reconstructing low-latitude ocean glacial-interglacial changes in sea surface temperatures (SST). We report herein a comparison of Mg/Ca measurements on sample pairs (n = 20) of two G. ruber (white) morphotypes (G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.)) from surface and downcore samples of the western Pacific and Indian Oceans. G. ruber s.s. refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide, high arched aperture, whereas G. ruber s.l. refers to a more compact test with a diminutive final chamber and small aperture. The G. ruber s.s. specimens generally show significantly higher Mg/Ca ratios compared to G. ruber s.l. Our results from the Mg/Ca ratio analysis suggest that G. ruber s.l. specimens precipitated their shells in slightly colder surface waters than G. ruber s.s. specimens. This conclusion is supported by the differences in delta18O and delta13C values between the two morphotypes. Although it is still unclear if these two morphotypes represent phenotypic variants or sibling species, our findings seem to support the hypothesis of depth and/or seasonal allopatry within a single morphospecies.

Geochemistry of igneous rock samples of ODP Site 125-786

A morphologically complex igneous basement was penetrated at Leg 125 Site 786 beneath approximately 100 m of Eocene-Pleistocene sediments at 31°52.45 'N, 141°13.59'E in a 3082-m water depth. The site is located on the forearc basement high (FBH) of the Izu-Bonin (Ogasawara) Arc. In the broadest terms, the sequence in Hole 786B consists of a basal sheeted dike complex, heavily mineralized in places, with overlying pillow lavas giving way to a complex and repeated sequence of interlayered volcanic breccias and lava flows with some thin sedimentary intervals. The sequence has been further cut by dikes or sills, particularly of high-Ca and intermediate-Ca boninite, and is locally strongly sheared by faulting. The whole basement has been covered with middle Eocene-early Pleistocene sediments. A monomict breccia forms the shallowest portion of Hole 786B and a polymict breccia having Mn-oxide-rich clast coatings and matrix forms the deepest part of Hole 786A (-100-160 mbsf). The basement is tectonized in some places, and a mineralized stockwork is present in the deepest part of Hole 786B. A wide variety of rock types form this basement, ranging from mafic to silicic in character and including high-, intermediate-, and low-Ca boninites, intermediate- and low-Ca bronzite andesites, andesite, dacite, and rhyolite groups. Intragroup and intergroup relationships are complicated in detail, and several different upper mantle source(s) probably were involved. A significant role for orthopyroxene-clinopyroxene-plagioclase fractionation is indicated in the mafic-intermediate groups, and the most probable complementary cumulates should be noritic gabbros. Many overall similarities but some subtle differences are noted between the igneous basement at Site 786 and the subaerial outcrops of the FBH to the south in the type boninite locality of Chichijima. Both suites were derived by hydrous melting of a relatively shallow, refractory (harzburgitic) upper mantle source. These Bonin forearc basement rocks are similar in many respects to those of Eocene-Oligocene age now forming the forearc of the Marianas at Leg 60 Site 458 and on Guam. In sharp distinction, the geochemistry of the Eocene-Pleistocene ash sequences overlying the Bonin FBH must have been derived from a very different upper mantle source, implying considerable across-strike differences in sub-arc mantle composition.

Rb-Sr isotope systematics and Sr/Ca-Ba/Ca ratios at DSDP Hole 89-462A

Four samples of Nauru Basin basalts (Cores 94 to 109 of Hole 462A, sub-bottom depth 1077-1209 m) have 87Sr/86Sr ratios in the range 0.7037 to 0.7038, which is distinctly higher than the ratios of N-type MORB. The Rb contents of the samples are depleted in comparison with those of MORB and ocean-island basalts. These chemical and isotopic characteristics are identical to those of the basalts previously drilled during Leg 61 (Cores 75 to 90 of Hole 462A), and are explained in terms of inhomogeneity of the source region in the mantle or later alteration effects. Sr/Ca-Ba/Ca systematics of 15 samples from Cores 462A-94 to 462A-109 and 14 samples from Cores 462A-75 to 462A-90 suggest that the Nauru Basin basalts are derived from a mantle peridotite by 20 to 30% partial melting with subsequent Plagioclase crystallization.

Radiocarbon ages, carbon and oxygen isotopes and Mg/Ca and Sr/Ca ratios of Globigerinoides in sediment core MD99-2203, Cape Hatteras

This study presents high-resolution foraminiferal-based sea surface temperature, sea surface salinity and upper water column stratification reconstructions off Cape Hatteras, a region sensitive to atmospheric and thermohaline circulation changes associated with the Gulf Stream. We focus on the last 10,000 years (10 ka) to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability. We observed opposite evolutions between the conditions off Cape Hatteras and those south of Iceland, known today for the North Atlantic Oscillation pattern. We interpret the temperature and salinity changes in both regions as co-variation of activities of the subtropical and subpolar gyres. Around 8.3 ka and 5.2-3.5 ka, positive salinity anomalies are reconstructed off Cape Hatteras. We demonstrate, for the 5.2-3.5 ka period, that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north. A northward displacement of the Gulf Stream, blocking the southbound low-salinity flow, concomitant to a reduced Meridional Overturning Circulation is the most likely scenario. Finally, wavelet transform analysis revealed a 1000-year period pacing the d18O signal over the early Holocene. This 1000-year frequency band is significantly coherent with the 1000-year frequency band of Total Solar Irradiance (TSI) between 9.5 ka and 7 ka and both signals are in phase over the rest of the studied period.