Oligocene plaktonic foraminifera of the Indian Ocean ODP Leg 115 sites

We drilled 13 holes on Ocean Drilling Program Leg 115 in the Indian Ocean and recovered Paleogene sediments that consisted primarily of pelagic components. Planktonic foraminifer assemblages displayed high diversity throughout the Paleogene from the late Paleocene to the Oligocene/Miocene boundary and consist of predominantly warm-water species. Faunas of middle Eocene age are remarkably well represented. Biostratigraphic assignment was, however, very difficult because of the turbiditic character of most of the Paleogene sediments. Reworking is a constant feature of the middle Eocene through early Oligocene planktonic faunas, with reworked faunas frequently overwhelming the younger ones. Preservation within turbidites ranges from excellent to very poor to total destruction of planktonic foraminifers. A major dissolution episode is recorded in the interval that spans most of the late Eocene through the early Oligocene, especially at the deeper sites where the source area was probably well below the lysocline. Redeposition decreases markedly by the mid-Oligocene, but it is only by late Oligocene Zone P22 that normal sedimentation resumes and/or redeposition decreases even at the most affected sites (such as Hole 709C). Comparison with other sites drilled previously in the Indian Ocean reveals that mixed assemblages were already known for sediments from the Mascarene Plateau-Seychelles Bank and surrounding basins during that time span. Because of the disturbances that characterize Paleogene deposits, hiatuses are difficult to detect; nevertheless, a hiatus of less local importance, spanning Subzone P21b, was detected in three holes at different water depths.

Planktonic foraminifera and stable isotope record of ODP Site 162-980

Detailed faunal, isotopic, and lithic marine records provide new insight into the stability and climate progression of the last interglacial period, Marine Isotope Stage (MIS) 5, which peaked approximately 125,000 years ago. In the eastern subpolar North Atlantic, at the latitude of Ireland, interglacial warmth of the ice volume minimum of substage 5e (MIS 5e) lasted ~10,000 years (10 ka) and its demise occurred in two cooling steps. The first cooling step marked the end of the climatic optimum, which was 2-3 ka long. Minor ice rafting accompanied each cooling step; the second, larger, step encompassing cold events C26 and C25 was previously identified in the northwestern Atlantic. Approximately 4 °C of cooling occurred between peak interglacial warmth and C25, and the region experienced an additional temporary cooling of at least 1-2 °C during C24, a cooling event associated with widespread ice rafting in the North Atlantic. Beginning with C24, MIS 5 was characterized by oscillations of at least 1-2 °C superimposed on a generally cool baseline. The results of this study imply that the marine climatic optimum of the last interglacial was shorter than previously thought. The finding that the eastern subpolar North Atlantic cooled significantly before C24 reconciles terrestrial evidence for progressive climate deterioration at similar and lower latitudes with marine conditions. Our results also demonstrate a close association between modest ice rafting, cooling, and deep ocean circulation even during the peak of MIS 5e and in the earliest stages of ice growth.

Temperature and d18O measurements from different ODP Sites at the shelf and upper slope of the Peru Margin

The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos et al. 1995 doi:10.1130/G20992.1] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use d18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater d18O values in the porewater, which is supported by d18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister et al. 2007, doi:10.1111/j.1365-3091.2007.00870.x], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate-methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate-methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.

Pb isotopes in individual sand-sized (>150 µm) ice-rafted feldspars deposited at subpolar North Atlantic Ocean Deep Sea Drilling Project Site 611 during the late Pliocene (Marine Isotope Stages, MIS, G6 through 100)

The onset of abundant ice-rafted debris (IRD) deposition in the Nordic Seas and subpolar North Atlantic Ocean 2.72 millions of years ago (Ma) is thought to record the Pliocene onset of major northern hemisphere glaciation (NHG) due to a synchronous advance of North American Laurentide, Scandinavian and Greenland ice-sheets to their marine calving margins during marine isotope stage (MIS) G6. Numerous marine and terrestrial records from the Nordic Seas region indicate that extensive ice sheets on Greenland and Scandinavia increased IRD inputs to these seas from 2.72 Ma. The timing of ice-sheet expansion on North America as tracked by IRD deposition in the subpolar North Atlantic Ocean, however, is less clear because both Europe and North America are potential sources for icebergs in this region. Moreover, cosmogenic-dating of terrestrial tills on North America indicate that the Laurentide Ice Sheet did not extend to ~39°N until 2.4 ±0.14 Ma, at least 180 ka after the onset of major IRD deposition at 2.72 Ma. To address this problem,we present the first detailed analysis of the geochemical provenance of individual sand-sized IRD deposited in the subpolar North Atlantic Ocean between MIS G6 and 100 (~2.72-2.52 Ma). IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single ice-rafted (>150 mm) feldspar grains. To track when an ice-rafting setting consistent with major NHG first occurred in the North Atlantic Ocean during the Pliocene intensification of NHG (iNHG), we investigate when the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb) of feldspars deposited at DSDP Site 611 first resembles that determined for IRD deposited at this site during MIS 100, the oldest glacial for which there exists convincing evidence for widespread glaciation of North America. Whilst Quaternary-magnitude IRD fluxes exist at Site 611 during glacials from 2.72 Ma, we find that the provenance of this IRD is not constant. Instead, we find that the Pb isotope composition of IRD at our study site is not consistent with major NHG until MIS G2 (2.64 Ma). We hypothesise that IRD deposition in the North Atlantic Ocean prior to MIS G2 was dominated by iceberg calving from Greenland and Scandinavia. We further suggest that the grounding line of continental ice on Northeast America may not have extended onto the continental shelf and calved significant numbers of icebergs to the North Atlantic Ocean during glacials until 2.64 Ma.