Late Quaternary siliciclastic/carbonate sedimentation in the Cabricorn Channel

A model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial-interglacial cycle in the Capricorn Channel, southern Great Barrier Reef (GBR). Stable isotope ratios, grainsize, carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level (mbsl). Results show variations in the flux of terrigenous, neritic and pelagic sediments to the continental slope over the last sea level cycle. During the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl. The percentages of quartz and feldspar in the cores decreased with water depth, while the percentage of clay increased. X-ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the Fitzroy River, which debouched directly into the northwest sector of the Capricorn Channel at this time. The cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite (HMC) during the glacial. The influx of HMC and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial, and also from HMC ooids precipitated at the head of the Capricorn Channel at this time. Mass accumulation rates (MARs) are high (13.5 g/cm**/kyr) during the glacial and peak at ~20 g/cm** 3/kyr in the early transgression (16-14 ka BP). MARs then decline with further sea level rise as the Fitzroy River mouth retreats from the edge of the continental shelf after 13.5 ka BP. MARs remain low (4 g/cm**3/kyr) throughout the Holocene highstand. Data for the Holocene highstand indicate there is a reduction in siliciclastic influx to the Capricorn Channel with little quartz and feldspar below 350 mbsl. However, fine-grained fluvial sediments, presumably from the Fitzroy River, were still accumulating on the mid slope down to 2000 mbsl. The proportion of pelagic calcite in the core tops increases with water depth, while HMC decreases, and is present only in trace amounts in cores below 1500 mbsl. The difference in the percentage of HMC in the deeper cores between the glacial and Holocene may reflect differences in supply or deepening of the HMC lysocline during the glacial. Sediment accumulation rates also vary between cores in the Capricorn Channel and do not show the expected exponential decrease with depth. This may be due to intermediate or deep water currents reworking the sediments. It is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments. Comparison of the Capricorn Channel MARs with those for other mixed carbonate/siliciclastic provinces from the northeast margin of Australia indicates that peak MARs in the early transgression in the Capricorn Channel precede those from the central GBR and south of Fraser Island. The difference in the timing of the carbonate and siliciclastic MAR peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces. The only common trend in the MARs from the northeast margin of Australia is the near synchronicity of the carbonate and siliciclastic MAR peaks in individual sediment cores, which supports a coeval sedimentation model.

Accumulation and composition of terrigenous material on Ceara rise over the past 55 Ma

Ceara Rise, located east the Amazon River mouth, is covered with a thick blanket of pelagic carbonate and hemipelagic terrigenous sediment. The terrigenous component has been extracted from 57 bulk sediment samples at Ocean Drilling Program (ODP) Sites 925 and 929 on Ceara Rise to obtain a Cenozoic record of riverine discharge from northern South America. From the early Eocene to early Miocene (55-20 Ma), terrigenous accumulation was dominated by moderate amounts of generally large-grained, gray to green sediment especially depleted in elements that are enriched in post-Archaean shale (e.g. Cs, Th, Yb). However, pulsed inputs of relatively small-grained, gray to green terrigenous sediment less depleted in the above elements occurred in the late Eocene and Oligocene. The accumulation of terrigenous sediment decreased significantly until 16.5 Ma. In the middle Miocene (16.5-13 Ma), terrigenous accumulation was dominated by small amounts of small-grained, tan sediment notably depleted in Na and heavy rare earth elements. The accumulation rate of terrigenous sediment increased markedly from the latest Miocene (10 Ma) to the present day, a change characterized by deposition of gray-green sediment enriched in elements that are enriched in post-Archaean shale. Observed changes in terrigenous sediment at Ceara Rise record tectonism and erosion in northern South America. The Brazil and Guyana shields supplied sediment to the eastern South American margin until the middle Miocene (20-16.5 Ma) when a period of thrusting, shortening and uplift changed the source region, probably first to highly weathered and proximal Phanerozoic sediments. By the late Miocene (9 Ma), there was a transcontinental connection between the Andes and eastern South America. Weathering products derived from the Andes have increasingly dominated terrigenous deposition at Ceara Rise since the Late Miocene and especially since the late Pliocene.

Ice-rafted debris in ODP Holes 177-1090D and 188-1165B

Constraining the nature of Antarctic Ice Sheet (AIS) response to major past climate changes may provide a window onto future ice response and rates of sea level rise. One approach to tracking AIS dynamics, and differentiating whole system versus potentially heterogeneous ice sheet sector changes, is to integrate multiple climate proxies for a specific time slice across widely distributed locations. This study presents new iceberg-rafted debris (IRD) data across the interval that includes Marine Isotope Stage 31 (MIS 31: 1.081-1.062 Ma, a span of ~19 kyr; Lisiecki and Raymo, 2005), which lies on the cusp of the mid-Brunhes climate transition (as glacial cycles shifted from ~41,000 yr to ~100,000 yr duration). Two sites are studied - distal Ocean Drilling Program (ODP) Leg 177 Site 1090 (Site 1090) in the eastern subantarctic sector of the South Atlantic Ocean, and proximal ODP Leg 188 Site 1165 (Site 1165), near Prydz Bay, in the Indian Ocean sector of the Antarctic margin. At each of these sites, MIS 31 is marked by the presence of the Jaramillo Subchron (0.988-1.072 Ma; Lourens et al., 2004) which provides a time-marker to correlate these two sites with relative precision. At both sites, records of multiple climate proxies are available to aid in interpretation. The presence of IRD in sediments from our study areas, which include garnets indicating a likely East Antarctic Ice Sheet (EAIS) origin, supports the conclusion that although the EAIS apparently withdrew significantly over MIS 31 in the Prydz Bay region and other sectors, some sectors of the EAIS must still have maintained marine margins capable of launching icebergs even through the warmest intervals. Thus, the EAIS did not respond in complete synchrony even to major climate changes such as MIS 31. Further, the record at Site 1090 (supported by records from other subantarctic locations) indicates that the glacial MIS 32 should be reduced to no more than a stadial, and the warm interval of Antarctic ice retreat that includes MIS 31 should be expanded to MIS 33-31. This revised warm interval lasted about 52 kyr, in line with several other interglacials in the benthic d18O records stack of Lisiecki and Raymo (2005), including the super-interglacials MIS 11 (duration of 50 kyr) and MIS 5 (duration of 59 kyr). The record from Antarctica-proximal Site 1165, when interpreted in accord with the record from ANDRILL-1B, indicates that in these southern high latitude sectors, ice sheet retreat and the effects of warming lasted longer than at Site 1090, perhaps until MIS 27. In the current interpretations of the age models of the proximal sites, ice sheet retreat began relatively slowly, and was not really evident until the start of MIS 31. In another somewhat more speculative interpretation, ice sheet retreat began noticeably with MIS 33, and accelerated during MIS 31. Ice sheet inertia (the lag-times in the large-scale responses of major ice sheets to a forcing) likely plays an important part in the timing and scale of these events in vulnerable sectors of the AIS.

Alkenone accumulation and SST reconstruction for the Benguela upwelling zone

A feature of Pliocene climate is the occurrence of "permanent El Niño-like" or "El Padre" conditions in the Pacific Ocean. From the analysis of sediment cores in the modern northern Benguela upwelling, we show that the mean oceanographic state off Southwest Africa during the warm Pliocene epoch was also analogous to that of a persistent Benguela "El Niño". At present these events occur when massive southward flows of warm and nutrient-poor waters extend along the coasts of Angola and Namibia, with dramatic effects on regional marine ecosystems and rainfall. We propose that the persistent warmth across the Pliocene in the Benguela upwelling ended synchronously with the narrowing of the Indonesian seaway, and the early intensification of the Northern Hemisphere Glaciations around 3.0-3.5 Ma. The emergence of obliquity-related cycles in the Benguela sea surface temperatures (SST) after 3 Ma highlights the development of strengthened links to high latitude orbital forcing. The subsequent evolution of the Benguela upwelling system was characterized by the progressive intensification of the meridional SST gradients, and the emergence of the 100 ky cycle, until the modern mean conditions were set at the end of the Mid Pleistocene transition, around 0.6 Ma. These findings support the notion that the interplay of changes in the depth of the global thermocline, atmospheric circulation and tectonics preconditioned the climate system for the end of the warm Pliocene epoch and the subsequent intensification of the ice ages.

Benthic foraminiferal assemblages and paleoproductivity reconstructions for the last 550 kyr of the Ontong Java Plateau, western equatorial Pacific

We analyzed foraminiferal and nannofossil assemblages and stable isotopes in samples from ODP Hole 807A on the Ontong Java Plateau in order to evaluate productivity and carbonate dissolution cycles over the last 550 kyr (kilo year) in the western equatorial Pacific. Our results indicate that productivity was generally higher in glacials than during interglacials, and gradually increased since MIS 13. Carbonate dissolution was weak in deglacial intervals, but often reached a maximum during interglacial to glacial transitions. Carbonate cycles in the western equatorial Pacific were mainly influenced by changes of deep-water properties rather than by local primary productivity. Fluctuations of the estimated thermocline depth were not related to glacial to interglacial alternations, but changed distinctly at ~280 kyr. Before that time the thermocline was relatively shallow and its depth fluctuated at a comparatively high amplitude and low frequency. After 280 kyr, the thermocline was deeper, and its fluctuations were at lower amplitude and higher frequency. These different patterns in productivity and thermocline variability suggest that thermocline dynamics probably were not a controlling factor of biological productivity in the western equatorial Pacific Ocean. In this region, upwelling, the influx of cool, nutrient-rich waters from the eastern equatorial Pacific or of fresh waters from rivers have probably never been important, and their influence on productivity has been negligible over the studied period. Variations in the inferred productivity in general are well correlated with fluctuations in the eolian flux as recorded in the northwestern Pacific, a proxy for the late Quaternary history of the central East Asian dust flux into the Pacific. Therefore, we suggest that the dust flux from the central East Asian continent may have been an important driver of productivity in the western Pacific.

Sedimentology of the Reykjanes Ridge, Southeast Greenland

Marine records from the Reykjanes Ridge indicate ice sheet variations and abrupt climate changes. One of these records, ice-rafted detritus (IRD), serves as a proxy for iceberg discharges that probably indicates ice sheet fluctuations. The IRD records suggest that iceberg discharge 68,000-10,000 yr B.P. happened more frequently than the 7000- to 10,000-yr spacing of the Heinrich events. An IRD peak 67,000 to 63,000 yr B.P. further suggests that the Middle Weichselian glaciation started about 12,000 yr earlier in the North Atlantic than in the Norwegian Sea. Several later IRD events, in contrast, correlate with Norwegian Sea IRD-rich layers and imply coeval ice sheet advances in the North Atlantic and the Norwegian Sea. Coccoliths in a core from the Reykjanes Ridge show distinct peaks in species that record occasional inflow of warm surface water during the last glaciation, as previously reported from the eastern Labrador Sea. High abundances of coccoliths, together with a decrease ofNeogloboquadrina pachydermasin. and relatively low delta18O values, imply enhanced advection of the North Atlantic Current 69,000-67,000 yr B.P., 56,000-54,000 yr B.P., 35,000-33,000 yr B.P., and 26,000-23,000 yr B.P. This advection provided a regional moisture source for extension of ice sheets onto the shelf. In contrast, most of the IRD events are characterized by cold polar surface water masses indicating rapid variations in ocean surface conditions.

Eolian and silica deposition of ODP sites in the central North Pacific

Sediments recovered at Ocean Drilling Program Sites 885/886 (central North Pacific Ocean at 44°41'N, 168°14'W and 44°41'N, 168°16'W, respectively) record eolian deposition during the Cenozoic and late Mesozoic. We constructed a record of eolian MAR, which is a proxy for aridity/humidity of the climate in the continental source area. Eolian fluxes are low during the Late Cretaceous through Eocene, reflecting humid conditions in the source area. During the Oligocene, more arid climates prevailed at the source area, as indicated by increased eolian accumulation. The "Diatom Dump", an interval of enhanced silica deposition mainly apparent in the northwest Pacific, is reflected in the record at Sites 885/886 by two- to fivefold higher opal fluxes compared with younger and older sediments. Increased eolian deposition starting at 3.5 Ma and culminating at 2-2.6 Ma are coincident with the onset of Northern Hemisphere glaciation. Sites 885/886 lie 10° north of sites examined previously for the history of eolian deposition in the central North Pacific and therefore allow enhanced understanding of the latitudinal variation of the wind system.

Geochemistry of Eocene/Oligocene sediments of ODP Hole 154-925A

Paleoproductivity, nutrient burial, and carbon cycling were investigated across the Eocene/Oligocene (E/O) boundary (begin to end; 36.9-32.7 Ma at ~40 kyr resolution, timescale of Shackleton et al. (1999, doi:10.1098/rsta.1999.0407) at Ocean Drilling Program Site 925 on the Ceara Rise in the western equatorial Atlantic (3040 m present water depth; 748.26-850.70 mbsf). Downcore bulk sediment records of biogenic barium, total reactive phosphorus, biogenic silica, and calcium carbonate are interpreted to represent export production, net nutrient burial, biogenic opal production, and inorganic carbon burial, respectively. The global positive excursion in d13C subsequent to the E/O boundary is recorded at Site 925. Export production appears to have been externally forced by orbital parameters at eccentricity frequencies during the study interval, based on spectral analysis of the biogenic barium and reactive phosphorus records. Biogenic silica production or preservation increased after the Eocene/Oligocene boundary to a higher baseline, although overall productivity and nutrient burial did not increase, based on barium and reactive phosphorus records. Thus, although absolute production did not increase at this site, a shift in relative abundance of siliceous versus carbonate productivity may have resulted in a change in relative organic carbon burial. This may have contributed to the positive excursion in global oceanic d13C subsequent to the Eocene/Oligocene boundary, although the silica maximum persists after the carbon isotope excursion ends.

Ichthyolith (fish tooth and denticle) counts from DSDP Site 91-596 and ODP Site 145-886

While the history of taxonomic diversification in open ocean lineages of ray-finned fish and elasmobranchs is increasingly known, the evolution of their roles within the open ocean ecosystem remains poorly understood. To assess the relative importance of these groups through time, we measured the accumulation rate of microfossil fish teeth and elasmobranch dermal denticles (ichthyoliths) in deep sea sediment cores from the North and South Pacific gyres over the past 85 million years. We find three distinct and stable open ocean ecosystem structures, each defined by the relative and absolute abundance of elasmobranch and ray-finned fish remains. The Cretaceous Ocean (pre-66 Ma), was characterized by abundant elasmobranch denticles, but low abundances of fish teeth. The Paleogene Ocean (66-20 Ma), initiated by the Cretaceous/Paleogene Mass Extinction, had nearly 4 times the abundance of fish teeth compared to elasmobranch denticles. This Paleogene Ocean structure remained stable during the Eocene greenhouse (50 Ma) and the Eocene-Oligocene glaciation (34 Ma), despite large changes in overall accumulation of both groups during those intervals, suggesting that climate change is not a primary driver of ecosystem structure. Dermal denticles virtually disappeared from open ocean ichthyolith assemblages about 20 Ma, while fish tooth accumulation increased dramatically in variability, marking the beginning of the Modern Ocean. Together, these results suggest that open ocean fish community structure is stable on long timescales, independent of total production and climate change. The timing of the abrupt transitions between these states suggests that the transitions may be due to interactions with other, non-preserved pelagic consumer groups.

Late Eocene to early Oligocene carbonate and barite accumulation rates in the Pacific Basin

The late Eocene through earliest Oligocene (40-32 Ma) spans a major transition from greenhouse to icehouse climate, with net cooling and expansion of Antarctic glaciation shortly after the Eocene/Oligocene (E/O) boundary. We investigated the response of the oceanic biosphere to these changes by reconstructing barite and CaCO3 accumulation rates in sediments from the equatorial and North Pacific Ocean. These data allow us to evaluate temporal and geographical variability in export production and CaCO3 preservation. Barite accumulation rates were on average higher in the warmer late Eocene than in the colder early Oligocene, but cool periods within the Eocene were characterized by peaks in both barite and CaCO3 accumulation in the equatorial region. We infer that climatic changes not only affected deep ocean ventilation and chemistry, but also had profound effects on surface water characteristics influencing export productivity. The ratio of CaCO3 to barite accumulation rates, representing the ratio of particulate inorganic C accumulation to Corg export, increased dramatically at the E/O boundary. This suggests that long-term drawdown of atmospheric CO2 due to organic carbon deposition to the seafloor decreased, potentially offsetting decreasing pCO2 levels and associated cooling. The relatively larger increase in CaCO3 accumulation compared to export production at the E/O suggests that the permanent deepening of the calcite compensation depth (CCD) at that time stems primarily from changes in deep water chemistry and not from increased carbonate production.

Paleoceanography on sediment core MD01-2416

IMAGES core MD01-2416 (51°N, 168°E) provides the first centennial-scale multiproxy record of Holocene variation in North Pacific sea-surface temperature (SST), salinity, and biogenic productivity. Our results reveal a gradual decrease in subarctic SST by 3-5 °C from 11.1 to 4.2 ka and a stepwise long-term decrease in sea surface salinity (SSS) by 2-3 p.s.u. Early Holocene SSS were as high as in the modern subtropical Pacific. The steep halocline and stratification that is characteristic of the present-day subarctic North Pacific surface ocean is a fairly recent feature, developed as a product of mid-Holocene environmental change. High SSS matched a salient productivity maximum of biogenic opal during Bølling-to-Early Holocene times, reaching levels similar to those observed during preglacial times in the warm mid-Pliocene prior to 2.73 Ma. Similar productivity spikes marked every preceding glacial termination of the last 800 ka, indicating recurrent short-term events of mid-Pliocene-style intense upwelling of nutrient-rich Pacific Deepwater in the Pleistocene. Such events led to a repeated exposure of CO2-rich deepwater at the ocean surface facilitating a transient CO2 release to the atmosphere, but the timing and duration of these events repudiate a long-term influence of the subarctic North Pacific on global atmospheric CO2 concentration.

Eolian mass-accumulation rates and grain size at DSDP Sites 86-576 and 86-578

The mass-accumulation rate and grain size of the total eolian component of North Pacific pelagic clays at Deep Sea Drilling Project Sites 576 and 578 have been used to evaluate changes in eolian sedimentation and the intensity of atmospheric circulation that have occurred during the past 70 m.y. Eolian deposition, an indicator of source area aridity, was low in the Paleocene, Eocene, and Oligocene, apparently reflecting the humid environments of that time as well as the lack of glacial erosion products. A general increase in eoiian accumulation in the Miocene apparently reflects the relative increase in global aridity during the latter part of the Cenozoic. A dramatic increase in eolian accumulation rates in the Pliocene reflects the increased aridity and availability of glacial erosion products associated with Northern Hemisphere glaciation 2.5 m.y. ago. Eolian grain size, an indicator of wind intensity, suggests that Late Cretaceous wind strength was comparable to present-day wind strength. A sharp decrease in eolian grain size across the Paleocene/Eocene boundary is not readily interpreted, but may indicate a significant reduction in the intensity of atmospheric circulation at that time. Fine eolian grain size and low accumulation rates in the Eocene and early Oligocene are in agreement with low early Tertiary thermal gradients and less vigorous atmospheric circulation. Large increases in grain size during the Oligocene, mid-to-late Miocene, and Pliocene appear to be a response to steepening thermal gradients resulting from increasing polar isolation.

Sedimentology and geochemistry of ODP Site 184-1144

We analyzed sediment from Ocean Drilling Program (ODP) Site 1144 in the northern South China Sea to examine the weathering response of SE Asia to the strengthening of the East Asian Monsoon (EAM) since 14 ka. Our high-resolution record highlights the decoupling between continental chemical weathering, physical erosion and summer monsoon intensity. Mass accumulation rates, Ti/Ca, K/Rb, hematite/goethite and 87Sr/86Sr show sharp excursions from 11 to 8 ka, peaking at 10 ka. Clay minerals show a shorter-lived response with a higher kaolinite/(illite + chlorite) ratio at 10.7-9.5 ka. However, not all proxies show a clear response to environmental changes. Magnetic susceptibility rises sharply between 12 and 11 ka. Grain-size becomes finer from 14 to 10 ka and then coarsens until ~7 ka, but is probably controlled by bottom current flow and sealevel. Sr and Nd isotopes show that material is dominantly eroded from Taiwan with a lesser flux from Luzon, while clay mineralogy suggests that the primary sources during the Early Holocene were reworked via the shelf in the Taiwan Strait, rather than directly from Taiwan. Erosion was enhanced during monsoon strengthening and caused reworking of chemically weathered Pleistocene sediment largely from the now flooded Taiwan Strait, which was transgressed by ~8 ka, cutting off supply to the deep-water slope. None of the proxies shows an erosional response lasting until ~6 ka, when speleothem oxygen isotope records indicate the start of monsoon weakening. Although more weathered sediments were deposited from 11 to 8 ka when the monsoon was strong these are reworked and represent more weathering during the last glacial maximum (LGM) when the summer monsoon was weaker but the shelves were exposed.

Uranium radionuclides and organic biomarkers in sediment core RC27-42

Uranium series radionuclides and organic biomarkers, which represent major groups of planktonic organisms, were measured in western Arabian Sea sediments that span the past 28 ka. Variability in the past strength of the southwest and northeast monsoons and its influence on primary productivity, sea surface temperature (SST), and planktonic community structure were investigated. The average alkenone-derived SST for the last glacial period was ~3°C lower than that measured for the Holocene. Prior to the deglacial, the lowest SSTs coincide with the highest measured fluxes of organic biomarkers, which represent primarily a planktonic suite of diatoms, coccolithophorids, dinoflagellates, and zooplankton. We propose that intensification of winter northeast monsoon winds during the last glacial period resulted in deep convective mixing, cold SSTs and enhanced primary productivity. In contrast, postdeglacial (<17 ka) SSTs are warmer during times in which biomarker fluxes are high. Associated with this transition is a planktonic community structure change, in which the ratio of the average cumulative flux of diatom biomarkers to the cumulative flux of coccolithophorid biomarkers is twice as high during the deglacial and Holocene than the average ratio during the last glacial period. We suggest that this temporal transition represents a shift from a winter northeast monsoon-dominated (pre-17 ka) to a summer southwest monsoon-dominated (post-17 ka) wind system.