Benthic and planktonic foraminifera of sediment core GIK17957-2

A detailed age model for core 17957-2 of the southern South China Sea was developed based on delta18O, coarse fraction, magnetostratigraphy, and biostratigraphy for the last 1500 kyr. The delta18O record has clear ~100-kyr cycles after the Mid-Pleistocene Revolution (MPR) at the entrance of marine isotopic stage (MIS) 22. Planktonic foraminifera responded to the MPR immediately, showing the increased sea surface temperature (SST) and dissolution after the MPR. Benthic foraminifera did not respond to it until the Brunhes/Matuyama boundary. Since the MPR, the depth of thermocline gradually became shallower until MISs 6-5. This major change within MISs 6-5 was also reflected in the decreased SSTs and increased productivity and Deep Water Mass. Thus two major Pleistocene paleoceanographic changes were found: One was around the MPR; the other occurred within MISs 6-5, which speculatively might be ascribed to the reorganization of surface and deep circulation, possibly induced by tectonic forces.

Multi-proxy analyses of sediment cores from the Pakistan continental margin

Late Holocene laminated sediments from a core transect centred in the oxygen minimum zone (OMZ) impinging at the continental slope off Pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years. High SW-monsoon-controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon (TOC) and redox-sensitive elements (Ni, V), as well as by a low-diversity, high-abundance benthic foraminiferal Buliminacea association and high abundance of the planktonic species Globigerina bulloides indicative of upwelling conditions. Surface-water productivity was strongest during SW monsoon maxima. Stable OMZ conditions (reflected by laminated sediments) were found also during warm interstadial events (Preboreal, Bølling-Allerød, and Dansgaard-Oeschger events), as well as during peak glacial times (17-22.5 ka, all ages in calendar years). Sediment mass accumulation rates were at a maximum during the Preboreal and Younger Dryas periods due to strong riverine input and mobilisation of fine-grained sediment coinciding with rapid deglacial sea-level rise, whereas eolian input generally decreased from glacial to interglacial times. In contrast, the occurrence of bioturbated intervals from 7 to 10.5 ka (early Holocene), in the Younger Dryas (11.7-13 ka), from 15 to 17 ka (Heinrich event 1) and from 22.5 to 25 ka (Heinrich event 2) suggests completely different conditions of oxygen-rich bottom waters, extremely low mass and organic carbon accumulation rates, a high-diversity benthic fauna, all indicating lowered surface-water productivity. During these intervals the OMZ was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods. The abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the North Atlantic Heinrich events and dust peaks in the Tibetan Loess records. The correlation of the monsoon-driven OMZ variability in the Arabian Sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale.

Interstitial water d13C and sediment geochemistry of ODP Leg 166 sites

A review of interstitial water samples collected from Sites 1003-1007 of the Bahamas Transect along with a shore-based analysis of oxygen and carbon isotopes, minor and trace elements, and sediment chemistry are presented. Results indicate that the pore-fluid profiles in the upper 100 meters below seafloor (mbsf) are marked by shifts between 20 and 40 mbsf that are thought to be caused by changes in sediment reactivity, sedimentation rates, and the influence of strong bottom currents that have been active since the late Pliocene. Pore-fluid profiles in the lower Pliocene-Miocene sequences are dominated by diffusion and do not show significant evidence of subsurface advective flow. Deeper interstitial waters are believed to be the in situ fluids that have evolved through interaction with sediments and diffusion. Pore-fluid chemistry is strongly influenced by carbonate recrystallization processes. Increases in pore-fluid Cl- and Na+ with depth are interpreted to result mainly from carbonate remineralization reactions that are most active near the platform margin. A lateral gradient in detrital clay content observed along the transect, leads to an overall lower carbonate reactivity, and enhances preservation of metastable aragonite further away from the platform margin. Later stage burial diagenesis occurs at slow rates and is limited by the supply of reactive elements through diffusion.

Authigenic uranium, mass accumulation rates, benthic d13C and SST during the last interglacial period of ODP Site 177-1094

Southern Ocean sediments reveal a spike in authigenic uranium 127,000 years ago, within the last interglacial, reflecting decreased oxygenation of deep water by Antarctic Bottom Water (AABW). Unlike ice age reductions in AABW, the interglacial stagnation event appears decoupled from open ocean conditions and may have resulted from coastal freshening due to mass loss from the Antarctic ice sheet. AABW reduction coincided with increased North Atlantic Deep Water (NADW) formation, and the subsequent reinvigoration in AABW coincided with reduced NADW formation. Thus, alternation of deep water formation between the Antarctic and the North Atlantic, believed to characterize ice ages, apparently also occurs in warm climates.

Mid-Pleistocene benthic foraminiferal record in the Southwest Pacific

Benthic foraminiferal faunas from three bathyal sequences provide a proxy record of oceanographic changes through the mid-Pleistocene transition (MPT) on either side of the Subtropical Front (STF), east of New Zealand. Canonical correspondence analyses show that factors related to water depth, latitude and climate cycles were more significant than oceanographic factors in determining changes in faunal assemblage composition over the last 1 Ma. Even so, mid-Pleistocene faunal changes are recognizable and can be linked to inferred palaeoceanographic causes. North of the largely stationary STF the faunas were less variable than to the south, perhaps reflecting the less extreme glacial-interglacial fluctuations in the overlying Subtropical Surface Water. Prior to Marine Isotope Stage (MIS) 21 and after MIS 15, the northern faunas had fairly constant composition, but during most of the MPT faunal composition fluctuated in response to climate-related food-supply variations. Faunal changes through the MPT suggest increasing food supply and decreasing dissolved bottom oxygen. South of the STF, beneath Subantarctic Surface Water, mid-Pleistocene faunas exhibited strong glacial-interglacial fluctuations, inferred to be due to higher interglacial nutrient supply and lower oxygen levels. The most dramatic faunal change in the south occurred at the end of the MPT (MIS 17- 12). with an acme of Abditodentrix pseudothalmanni, possibly reflecting higher carbon flux and lower bottom oxygen. This study suggests that the mid-Pleistocene decline and extinction of a group of elongate, cylindrical deep-sea foraminifera may have been related to decreased bottom oxygen concentrations as aresult of slower deep-water currents.