Mineralogy and isotope chemistry of GISP2 and potential source areas

Samples of dust from the Greenland Ice Sheet Project 2 (GISP2) ice core, Summit, Greenland, dated within marine isotope stage 2 (between 23,340 and 26,180 calendar years B.P.) around the time of the coldest, local, last glacial temperatures, have been analyzed to determine their provenance. To accomplish this, we have compared them with approximately Coeval aeolian sediments (mostly loesses) sampled in possible source areas (PSAs) from around the northern hemisphere. The <5-µm grain-size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale, which was sufficiently fine grained to be transported over long distances. On the basis of comparison of the clay mineralogy and Sr, Nd and Pb isotope composition with ice dust and PSAs and assuming that we have sampled the most important PSAs, we have determined that the probable source area of these GISP2 dusts was in eastern Asia. The dust was not derived from either the midcontinental United States or the Sahara, two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling. Except for a brief period during an interstadial, when dust transport was exceptionally low (for glacial times) and had a mineralogical composition indicative of a slightly more southern provenance, the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size. This includes the high-dust period that correlates with the Heinrich 2 period of major iceberg discharge into the North Atlantic. Variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size.

Locations of geochemical samples and tephra from ODP Site 181-1123

Approximately one thousand sediment samples from ODP Site 1123 on the Chatham Rise, east of New Zealand, have been examined for inorganic elemental concentrations. ODP 1123 provides a record of sediment drift deposition under the Deep Western Boundary Current, the main inflow of deep water to the Pacific Ocean since the Early Oligocene, though a major hiatus spans the late Early Oligocene to the Early Miocene. Normalisation of the elemental concentrations by aluminium was used to allow for the effects of variable carbonate dilution. The elemental ratios were used as proxies for sediment composition and as palaeoceanographic indices. The samples were collected at a resolution designed to sample adequately any variation in elemental ratios at the scale of the Milankovitch orbital cycles. The sampled intervals span the Early Oligocene, Early Miocene, mid-Miocene and Late Pleistocene to Recent. Anomalous Si/Al, K/Al, Ti/Al values in the upper Pleistocene section, often associated with horizons of low carbonate, are attributed to tephras derived from North Island. Not all of the tephras detected geochemically had been detected visually in the cores. A total of 37 tephra events between 1.17 Ma BP and the present are recognised based on this and the shipboard investigations. The tephra events cluster at intervals of approximately 326 000 years (326 ka) perhaps due to variations in eruption frequency on North Island and/or to variations in the regional palaeowind intensity and direction. In the Late Pleistocene to Recent P/Al (inferred nutrient availability), percent calcium carbonate (%CaCO3) and Ba/Al (inferred productivity) varied regularly at a period of 40 000 years with these factors lagging minimum global ice volumes (interglacials). During the mid-Miocene CaCO3, Ba/Al, P/Al and Si/Al all gradually increased with %CaCO3 and P/Al showing regular 138 000-yr cyclicity and Ba/Al showing 44-ka cyclicity. Inferred productivity (Ba/Al) may have been rising in association with increasing nutrient availability (P/Al) at the same time as increased vigour of the Deep Western Boundary Current that was connected to a period of rapid ice-sheet growth in Antarctica. In the Early Miocene P/Al and Si/Al were much higher than subsequently and both %CaCO3 and P/Al exhibited 131 000-yr cycles. By far the highest nutrient levels and inferred productivity at this site apparently occurred during the Early Oligocene as revealed by long-term changes in P/Al and Si/Al. A progressive rise in K/Al, but stable Ti/Al from the Early Oligocene to the Recent probably indicates increased proportions of illite in the clay mineral fraction of the drift sediments caused by increased flux of debris from the Southern Alps.

Millenial-Scale stable oxygen isotope record of ODP Site 172-1058

We use the oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber (white) from Ocean Drilling Program Site 1058 in the subtropical northwestern Atlantic to construct a high-resolution (~800 year) climate record spanning the mid-Pleistocene climate transition (~410 ka to 1350 ka). We investigate whether or not millennial-scale instabilities in the proxy record are associated with the extent of continental glaciation. G. ruber d18O values display high-frequency fluctuations throughout the record, but the amplitude about mean glacial and interglacial d18O values increases at marine isotope stage (MIS) 22 (880 ka) and is highest during MIS 12. These observations support that millennial-scale climate instabilities are associated with ice sheet size. Time series analysis illustrates that these variations have significant concentration of spectral power centered on periods of ~10-12 ka and ~5 ka. The timing of these fluctuations agrees well, or coincides with, the periodicities of the second and fourth harmonics, respectively, of precessional forcing at the equator. An insolation-based origin of the millennial-scale instabilities would be independent of ice volume and explains the presence of these fluctuations before the mid-Pleistocene climate transition as well as during interglacial intervals (e.g., MIS 37 and 17). Because the amplitude of the millennial-scale variations increases during the mid-Pleistocene transition, feedback mechanisms associated with the growth of large, 100-ka-paced, polar ice sheets may be important amplifiers of regional surface water hydrographic changes.

Geochemistry on sediment core Co1014 from Rauer Group, Antarctica

The history of glacial advances and retreats of the East Antarctic ice sheet during the Holocene is not well-known, due to limited field evidence in both the marine and terrestrial realm. A 257-cm-long sediment core was recovered from a marine inlet in the Rauer Group, East Antarctica, 1.8 km in front of the present ice-sheet margin. Radiocarbon dating and lithological characteristics reveal that the core comprises a complete marine record since 4500 yr. A significant ice-sheet expansion beyond present ice margins therefore did not occur during this period.

Global compilation of last glacial maximum oxygen isotopic ratios for planktonic and benthic foraminifera

We use the fully coupled atmosphere-ocean three-dimensional model of intermediate complexity iLOVECLIM to simulate the climate and oxygen stable isotopic signal during the Last Glacial Maximum (LGM, 21 000 yr). By using a model that is able to explicitly simulate the sensor (d18O), results can be directly compared with data from climatic archives in the different realms. Our results indicate that iLOVECLIM reproduces well the main feature of the LGM climate in the atmospheric and oceanic components. The annual mean d18O in precipitation shows more depleted values in the northern and southern high latitudes during the LGM. The model reproduces very well the spatial gradient observed in ice core records over the Greenland ice-sheet. We observe a general pattern toward more enriched values for continental calcite d18O in the model at the LGM, in agreement with speleothem data. This can be explained by both a general atmospheric cooling in the tropical and subtropical regions and a reduction in precipitation as confirmed by reconstruction derived from pollens and plant macrofossils. Data-model comparison for sea surface temperature indicates that iLOVECLIM is capable to satisfyingly simulate the change in oceanic surface conditions between the LGM and present. Our data-model comparison for calcite d18O allows investigating the large discrepancies with respect to glacial temperatures recorded by different microfossil proxies in the North Atlantic region. The results argue for a trong mean annual cooling between the LGM and present (>6°C), supporting the foraminifera transfer function reconstruction but in disagreement with alkenones and dinocyst reconstructions. The data-model comparison also reveals that large positive calcite d18O anomaly in the Southern Ocean may be explained by an important cooling, although the driver of this pattern is unclear. We deduce a large positive d18Osw anomaly for the north Indian Ocean that contrasts with a large negative d18Osw anomaly in the China Sea between the LGM and present. This pattern may be linked to changes in the hydrological cycle over these regions. Our simulation of the deep ocean suggests that changes in d18Osw between the LGM and present are not spatially homogenous. This is supported by reconstructions derived from pore fluids in deep-sea sediments. The model underestimates the deep ocean cooling thus biasing the comparison with benthic calcite d18O data. Nonetheless, our data-model comparison support a heterogeneous cooling of few degrees (2-4°C) in the LGM Ocean.

Geochemistry, lithology, and bulk density of Cricket Flat paleosol samples

The high-resolution marine isotope climate record indicates pronounced global cooling during the Langhian (16-13.8 Ma), beginning with the warm middle Miocene climatic optimum and ending with significant Antarctic ice sheet expansion and the transition to "icehouse" conditions. Terrestrial paleoclimate data from this interval is sparse and sometimes conflicting. In particular, there are gaps in the terrestrial record in the Pacific Northwest during the late Langhian and early Serravallian between about 14.5 and 12.5 Ma. New terrestrial paleoclimate data from this time and region could reconcile these conflicting records. Paleosols are particularly useful for reconstructing paleoenvironment because the rate and style of pedogenesis is primarily a function of surface environmental conditions; however, complete and well-preserved paleosols are uncommon. Most soils form in erosive environments that are not preserved, or in environments such as floodplains that accumulate in small increments; the resulting cumulic soils are usually thin, weakly developed, and subject to diagenetic overprinting from subsequent soils. The paleosol at Cricket Flat in northeastern Oregon is an unusually complete and well-preserved paleosol from a middle Miocene volcanic sequence in the Powder River Volcanic Field. An olivine basalt flow buried the paleosol at approximately 13.8 ± 0.6 Ma, based on three 40Ar/39Ar dates on the basalt. We described the Cricket Flat paleosol and used its physical and chemical profile and micromorphology to assess pedogenesis. The Cricket Flat paleosol is an Ultisol-like paleosol, chemically consistent with a high degree of weathering. Temperature and rainfall proxies suggest that Cricket Flat received 1120 ± 180 mm precipitation y-1 and experienced a mean annual temperature of 14.5 ± 2.1 °C during the formation of the paleosol, significantly warmer and wetter than today. This suggests slower cooling after the middle Miocene climatic optimum than is seen in the existing paleosol record.

Age determination and stable isotope record of planktonic foraminifera from sediments of the Baffin Shelf

Stable isotopic values on planktonic foraminifera in a suite of cores from basins across the SE Baffin Shelf are used to extract a record of meltwater events during Termination I deglaciation. Resolution and Hatton basins lie on the SE Baffin Shelf at water depths > 500 m, seaward of major conduits for ice drainage from the eastern sector of the Laurentide Ice Sheet (LIS). Accelerator mass spectrometry 14C dates are used to constrain our chronology of events in ten cores. In Resolution Basin, three cores have 14C AMS dates on foraminifera of > 20 ka at their bases; whereas Hatton Basin cores terminate in sediments < 13 kyr. Sedimentation rates varied between 0.1 to 4.5 m/ka. Stable oxygen and carbon isotopic ratios were obtained on 146 samples of the planktonic foraminifera Neogloboquadrina pachyderma (Ehrenberg) sinistral, from seven of the ten cores. No evidence was found to indicate that test morphology or size affected delta18O. Between 7 and 13.5 ka the surface water on the shelf was on average 1 per mil lower than the open ocean signal. Significant temporal variations were found in both delta18O and delta13C. Evidence for significant low delta18O events occurred between 13 and 8 ka. The delta13C record from the planktonic foraminifera suggests a threefold division of events between 13 and 7 ka, with positive values between 10.8 and 13.0 ka, negative values between 9 and 10.8 ka, and positive values from 7 to 9 ka. The delta18O data suggest the presence of meltwater on the shelf some 3,000 years prior to the first late glacial dates on terrestrial deglaciation (at circa 10.4 ka). "Hudson Strait must be the real key to the importance of the calving process during deglaciation, because it is potentially the largest marine outlet for the Laurentide Ice Sheet and because it leads into the very center of the ice sheet.....the rates of calving through Hudson Strait during the period of initial ?18O rise unfortunately are unknown." W. F. Ruddiman (1987, p. 151)

Oxygen and Carbon stable isotope ratios for Site U1334

The Oligocene-Miocene transition (OMT) (~23 Ma) is interpreted as a transient global cooling event, associated with a large-scale Antarctic ice sheet expansion. Here we present a 2.23 Myr long high-resolution (~3 kyr) benthic foraminiferal oxygen and carbon isotope (d18O and d13C) record from Integrated Ocean Drilling Program Site U1334 (eastern equatorial Pacific Ocean), covering the interval from 21.91 to 24.14 Ma. To date, five other high-resolution benthic foraminiferal stable isotope stratigraphies across this time interval have been published, showing a ~1 per mil increase in benthic foraminiferal d18O across the OMT. However, these records are still few and spatially limited and no clear understanding exists of the global versus local imprints. We show that trends and the amplitudes of change are similar at Site U1334 as in other high-resolution stable isotope records, suggesting that these represent global deep water signals. We create a benthic foraminiferal stable isotope stack across the OMT by combining Site U1334 with records from ODP Sites 926, 929, 1090, 1264, and 1218 to best approximate the global signal. We find that isotopic gradients between sites indicate interbasinal and intrabasinal variabilities in deep water masses and, in particular, note an offset between the equatorial Atlantic and the equatorial Pacific, suggesting that a distinct temperature gradient was present during the OMT between these deep water masses at low latitudes. A convergence in the d18O values between infaunal and epifaunal species occurs between 22.8 and 23.2 Ma, associated with the maximum d18O excursion at the OMT, suggesting climatic changes associated with the OMT had an effect on interspecies offsets of benthic foraminifera. Our data indicate a maximum glacioeustatic sea level change of ~50 m across the OMT.

Benthic foraminifers in Mesozoic and Cenozoic sediments of the southwestern Atlantic

Benthic foraminiferal assemblages in Mesozoic and Cenozoic sediments were studied at Sites 511, 512, 513, and 514 drilled during Leg 71 in the southwestern Atlantic on the Maurice Ewing Bank and in the Argentine Basin. Benthic foraminifers in almost all stratigraphic subdivisions of Sites 511 and 512 reflect the gradual subsidence of the Falkland Plateau from shelf depths in the Barremian-Albian, when a semiclosed basin with restricted circulation of water masses and anaerobic conditions existed, to lower bathyal depths in the Late Cretaceous and Cenozoic, with an abrupt acceleration at the boundary of Lower and Upper Cretaceous. The composition, distribution, and preservation of Late Cretaceous assemblages of benthic foraminifers suggest considerable fluctuations of the foraminiferal lysocline and the CCD. This is evidenced by dissolution facies and foraminiferal assemblages in which agglutinated and resistant calcareous forms predominated during high stands of the CCD and by calcareous facies in which rich assemblages of calcareous species predominated during low stands. The highest position of the CCD on the Plateau (less than 1500-2000 m) was in the late Cenomanian, Turonian, and Coniacian. In the Santonian and Campanian the CCD was at depths below 1500-2000 meters. At the end of the Campanian the CCD shifted again to depths comparable with those of Cenomanian and Turonian time. In the latest Campanian and the Maestrichtian the CCD was low and nanno-foraminiferal oozes with a rich assemblage of benthic foraminifers accumulated. Foraminiferal assemblages at Sites 513 and 514 in the Argentine Basin also testify to oceanic subsidence from lower bathyal depths in the Oligocene to abyssal ones at present. This process was complicated by the influence of geographical migrations of the Polar Front caused by extensions of the ice sheet in the Antarctic after the opening of the Drake Passage during the Oligocene. In Mesozoic and Cenozoic deposits of the Falkland Plateau and the Argentine Basin seven assemblages of benthic foraminifers were distinguished by age: early-middle Albian, middle-late Albian, Late Cretaceous (including four groups), middle Eocene, late Eocene-early Miocene, middle-late Miocene, and Pliocene-Quaternary. The Albian assemblages contain many species common to the foraminiferal fauna of the Austral Biogeographical Province. The Late Cretaceous assemblage contains, along with Austral species, species common to foraminifers of North America, Western Europe, the Russian platform, and the south of the U.S.S.R. Deep-sea cosmopolitan species prevail in Cenozoic assemblages.