Ages at DSDP Site 86-577

A synthesis of paleomagnetic and calcareous nannofossil stratigraphies for the sedimentary sequences recovered at Deep Sea Drilling Project (DSDP) Site 577 on the Shatsky Rise is presented. Numerical ages are estimated for a series of nannofossil datum levels from the late Maestrichtian to middle Eocene period ( about 68 to about 52 m.y. ago) and the late Cenozoic (last about 5 m.y.). Absolute age control is obtained on the basis of the revised geomagnetic polarity time scale of. The results are compared with various sets of data reported in the literature, in particular to magnetobiochronologies derived from marine sections accessible on land in Italy and from recent DSDP boreholes in the South Atlantic, and with the summary by Berggren et al. Although a number of minor discrepancies remain to be resolved, the remarkable general agreement of the data validates the basic concept of this approach to the elaboration of a calibrated geologic time scale.

Biosiliceous sedimentation at ODP Site 128-798

Upper Pliocene through Holocene sediments recovered at Site 798 in the Japan Sea (Oki Ridge) exhibit rhythmic variation in weight percent biogenic opal at intervals of ~5 m and periods equivalent to the 41-k.y. obliquity cycle. Variance at 17 and 100 k.y. is observed prior to 1.3 Ma. These cycles are also clearly defined by log data and correspond to clusters of decimeter-scale dark-colored sediment units alternating with clusters of light-colored units. Opal content varies between 3% and 22% between 0 and 1.3 Ma and from 3% to 43% between 1.3 and 2.6 Ma. Long-term opal accumulation rates average 1.8 g/cm**2/k.y. in the late Pliocene/early Pleistocene and decrease by about 60% at ~1.3 Ma. Rough calculations suggest that opal accumulation rates increased and terrigenous flux decreased during the Holocene relative to the last glacial period. Our age control is not yet sufficient to allow a similar analysis of the 41-k.y. cyclicity in opal content throughout the Pleistocene. Stable isotope results from planktonic foraminifers confirm previous suggestions of a strong surface-water freshening event during isotope stage 2; however, this episode appears to be unique during the Pleistocene. Benthic foraminifers are depleted in 18O during parts of glacial stages 2 and 6 relative to adjacent interglacials, suggesting unusual warming and/or freshening of deep waters. Collectively, the stable isotope and %opal data are consistent with continuing isolation of the Japan Sea during the Quaternary with important transitions occurring at 1.3, 0.7 to 1.0, and 0.2 to 0.3 Ma. Complex relationships among the stable isotope results, %opal data, and sediment characteristics such as color and organic and inorganic carbon content preclude development of a simple model to explain cyclical sedimentation. Opal maxima occur within both light and dark intervals and the processes that control surface-water productivity are at times decoupled from the factors that regulate deep-water dysaerobia. We suggest that water column overturn is controlled largely by regional atmospheric circulation that must also have an as yet poorly understood effect on surface-water fertility.

Sea surface temperature estimation for MIS 16 of IODP Site 306-U1313

Hudson Strait (HS) Heinrich Events, ice-rafting events in the North Atlantic originating from the Laurentide ice sheet (LIS), are among the most dramatic examples of millennial-scale climate variability and have a large influence on global climate. However, it is debated as to whether the occurrence of HS Heinrich Events in the (eastern) North Atlantic in the geological record depends on greater ice discharge, or simply from the longer survival of icebergs in cold waters. Using sediments from Integrated Ocean Drilling Program (IODP) Site U1313 in the North Atlantic spanning the period between 960 and 320 ka, we show that sea surface temperatures (SSTs) did not control the first occurrence of HS Heinrich(-like) Events in the sedimentary record. Using mineralogy and organic geochemistry to determine the characteristics of ice-rafting debris (IRD), we detect the first HS Heinrich(-like) Event in our record around 643 ka (Marine Isotope Stage (MIS) 16), which is similar as previously reported for Site U1308. However, the accompanying high-resolution alkenone-based SST record demonstrates that the first HS Heinrich(-like) Event did not coincide with low SSTs. Thus, the HS Heinrich(-like) Events do indicate enhanced ice discharge from the LIS at the end of the Mid-Pleistocene Transition, not simply the survivability of icebergs due to cold conditions in the North Atlantic.

Sedimentary records of ODP Site 198-1209 on Shatsky Rise

The accumulation of wind blown (eolian) dust in deep-sea sediments reflects the aridity/humidity conditions of the continental region supplying the dust, as well as the "gustiness" of the climate system. Detailed studies of Pleistocene glacial-interglacial dust fluxes suggest changes in accumulation rates corresponding to orbital variations in solar insolation (Milankovitch cycles). While the orbital cycles found in sedimentary archives of the Pleistocene are intricately related to glacial growth and decay, similar global orbital signals recognized in deep-sea sediments of early Paleogene age, the last major greenhouse interval ~65-45 million years ago, could not have been linked to the waxing and waning of large ice sheets. Thus orbital signals recorded in early Paleogene sediments must reflect some other climate response to changes in solar insolation. To explore the potential connection between orbital forcing and the climate processes that control dust accumulation, we generated a high-resolution dust record for ~58 Myr old sediments from Shatsky Rise (ODP Site 1209, paleolatitude ~15°N-20°N). The dust accumulation data provide the first evidence of a correlation between dust flux to the deep sea and orbital cyclicity during the early Paleogene, indicating dust supply responded to insolation forcing during the last major interval of greenhouse climate. Furthermore, the relative amplitude of the dust flux response during the early Paleogene greenhouse was comparable to that during icehouse climates. Thus, subtle variations in solar insolation driven by changes in Earth's orbit about the Sun may have had a similar impact on climate during intervals of overall warmth as they did during glacial-interglacial states.

Biomarker and XRD results for the last 3.4 Ma from IODP Site 306-U1313

Various types of abrupt/millennial-scale climate variability such as Dansgaard/Oeschger and Heinrich Events characterized the last glacial period. Over the last decade, a number of studies demonstrated that such millennial-scale climate variability was not limited to the last glacial but inherent to Quaternary climate. Here we review the occurrence and origin of millennial ice-rafting events in the North Atlantic during the late Pliocene and Pleistocene (last 3.4 Ma) with a special focus on North Atlantic Hudson Strait (HS) Heinrich(-like) Events. Besides a clear biomarker signature, we show that Heinrich Layers 5, 4, 2, and 1 in marine sediment cores from across the North Atlantic all bear the organic geochemical fingerprint of the Hudson area. Using this framework and combining previously published results, detailed investigations into the organic and inorganic chemistry of ice-rafted debris (IRD) found across the North Atlantic demonstrate that prior to MIS 16 (~ 650 ka) IRD in the North Atlantic did not originate from the Hudson area of northern Canada. The signature of this early IRD is distinctly different compared to that of HS Heinrich Layers. Rather ice-rafting events during the late Pliocene and early Pleistocene predominantly emanated from the calving of the Greenland and Fennoscandian ice sheets and possibly minor contributions from local ice streams from the North American and British ice sheets. Compared to North Atlantic HS Heinrich Events, these early Pleistocene IRD-events had a limited impact on surface water characteristics in the North Atlantic. North Atlantic HS Heinrich(-like) Events first occurred during MIS 16. At the same time, the dominant frequency in silicate-rich IRD accumulation shifted from the obliquity (41-ka) to a 100-ka frequency across the North Atlantic. Iceberg survivability or a change in iceberg trajectory likely did not control this change in IRD-regime. These results lend further support for the existing hypothesis that an increase in size (thickness) of the Laurentide ice sheet controls the occurrence of North Atlantic HS Heinrich Events, favoring an internal dynamic mechanism for their occurrence.