Phosphorus components and barite in untreated and in carbonate-free sediments of ODP Hole 199-1221C

We determined changes in equatorial Pacific phosphorus (µmol P/g) and barite (BaSO4; wt%) concentrations at high resolution (2 cm) across the Paleocene/Eocene (P/E) boundary in sediments from Ocean Drilling Program (ODP) Leg 199 Site 1221 (153.40 to 154.80 meters below seafloor [mbsf]). Oxide-associated, authigenic, and organic P sequentially extracted from bulk sediment were used to distinguish reactive P from detrital P. We separated barite from bulk sediment and compared its morphology with that of modern unaltered biogenic barite to check for diagenesis. On a CaCO3-free basis, reactive P concentrations are relatively constant and high (323 µmol P/g or ~1 wt%). Barite concentrations range from 0.05 to 5.6 wt%, calculated on a CaCO3-free basis, and show significant variability over this time interval. Shipboard measurements of P and Ba in bulk sediments are systematically lower (by ~25%) than shore-based concentrations and likely indicate problems with shipboard standard calibrations. The presence of Mn oxides and the size, crystal morphology, and sulfur isotopes of barite imply deposition in sulfate-rich pore fluids. Relatively constant reactive P, organic C, and biogenic silica concentrations calculated on a CaCO3-free basis indicate generally little variation in organic C, reactive P, and biogenic opal burial across the P/E boundary, whereas variable barite concentrations indicate significant changes in export productivity. Low barite Ba/reactive P ratios before and immediately after the Benthic Extinction Event (BEE) may indicate efficient nutrient burial, and, if nutrient burial and organic C burial are linked, high relative organic C burial that could temporarily drawdown CO2 at this site. This interpretation requires postdepositional oxidation of organic C because organic C to reactive P ratios are low throughout the section. After the BEE, higher barite Ba/reactive P ratios combined with higher barite Ba concentrations may imply that higher export productivity was coupled with unchanged reactive P burial, indicating efficient nutrient and possibly also organic C recycling in the water column. If the nutrient recycling is decoupled from organic C, the high export production could be indicative of drawdown of CO2. However, the observation that organic C burial is not high where barite burial is high may imply that either C sequestration was restricted to the deep ocean and thus occurred only on timescales of the deep ocean mixing or that postdepositional oxidation (burn down) of organic matter affected the sediments. The decoupling of barite and opal may result from low opal preservation or production that is not diatom based.

Geochemistry of carbon at DSDP Hole 61-462

Seven quartered sections of Pliocene to Mesozoic (Cenomanian) cores from the Nauru Basin contain primarily marine organic matter admixed with detectable amounts of terrigenous organic matter. The mixture is immature with respect to organic genesis. Chemical properties of this organic matter are compared with properties of other deep-ocean cores from DSDP sites in the central Pacific.

Mid-Pleistocene planktonic foraminiferal composition and IRD content in ODP Sites 162-980 and 162-984

We investigated surface and deep ocean variability in the subpolar North Atlantic from 1000 to 500 thousand years ago (ka) based on two Ocean Drilling Program (ODP) sites, Feni drift site 980 (55°29'N, 14°42'W) and Bjorn drift site 984 (61°25'N, 24°04'W). Benthic foraminiferal stable isotope data, planktic foraminiferal faunas, ice-rafted debris data, and faunally based sea-surface temperature estimates help test the hypothesis that oceanographic changes in the North Atlantic region were associated with the onset of the 100-kyr world during the mid-Pleistocene revolution. Based on percentage of Neogloboquadrina pachyderma (s) records from both sites, surface waters during interglacials and glacials were cooler in the mid-Pleistocene than during marine isotope stages (MIS) 5 and 6. In particular, interglaciations at Bjorn drift site 984 were significantly cooler. Faunal evidence suggests that the interglacial Arctic front shifted from a position between the two sites to a position northwest of Bjorn drift site 984 after ca. 610 ka. As during the late Pleistocene, we find faunal evidence for lagging surface warmth at most of the glacial initiations during the mid-Pleistocene. Each initiation is associated with high benthic d13C values that are maintained into the succeeding glaciation, which we term "lagging NADW production." These findings indicate that lagging warmth and lagging NADW production are robust features of the regional climate system that persist in the middle to late Pleistocene.

Consolidation and strength assessment of ODP Leg 123 sediments

Twenty-one samples, ranging in depth from 0 to 150 meters below seafloor (mbsf), were obtained from Leg 123 Sites 765 and 766. All samples were tested for Atterberg limits: 14 for laboratory vane shear strength and seven for uniaxial consolidation. Based on the determined Atterberg limits, along with shipboard measurements of water content, the sediment appears to be underconsolidated from 0 to 40 mbsf at Site 765 and from 0 to 80 mbsf at Site 766. Normal consolidation trends were observed for the sediments below these depths. Vane shear strengths, when compared with calculated values for a normally consolidated clay, indicate underconsolidated sediment at both sites. However, the use of Atterberg limit and vane shear strength data to assess consolidation state is complicated by the presence of silt-sized calcium carbonate in the form of nannofossil ooze. Thus, uniaxial-consolidation test data were analyzed to determine the overconsolidation ratios (OCR) and sediment compressibilities. OCR values were found to be less than one (underconsolidated) at both sites, using two separate methods of analysis.

Neogen carbonate and stable isotope record of ODP Hole 120-751A

Lower Miocene through upper Pleistocene benthic foraminifer assemblage records from Ocean Drilling Program Site 751 on the Southern Kerguelen Plateau (57°44'S, water depth 1634 m) were combined with benthic and planktonic foraminifer oxygen and carbon isotope records and high-resolution CaCO3 data from the same site. Implications for the Neogene productivity and paleoceanography of the southern Indian Ocean are discussed. We used distinctive features of the Miocene d18O and d13C curves for stratigraphic correlation. Coinciding with a lower middle Miocene hiatus from 14.2 to 13.4 Ma, there was a rapid increase in benthic d18O values by 1.2 per mil. This distinct increase occurs in middle Miocene benthic foraminifer oxygen isotope curves from all oceans. No major change, however, in benthic foraminifer faunal composition occurred in this period of growth of the Antarctic ice cap and cooling of deep ocean waters (14.9-14.2 Ma). A drastic change in benthic foraminifer faunas coincided with a hiatus from 8.4 to 5.9 Ma. Shortly after this hiatus, in the latest Miocene, the CaCO3 content of the sediments dropped from 75% to 0%. From that time ( 5.8 Ma) through the early Pliocene, Site 751 has been situated beneath a high biogenic siliceous productivity zone. Carbonate contents of upper Pliocene and Pleistocene sediments vary between 20% and 70%. The benthic foraminifer faunas in the uppermost Pliocene and lower Pleistocene reflect strong bottom current conditions, in contrast to those in the upper Pleistocene, which indicate calm sedimentation and high food supply. High d13C values of planktonic foraminifers compared with low values of benthic foraminifers suggest high primary productivity in the late Pleistocene. The changes in productivity were probably a result of latitudinal migration and meandering of the Polar Frontal Zone.

Stable carbon and oxygen isotope ratios of Planulina wuellerstorfi from sediments of the Sierra Leone Rise

Causes of change in deep water delta13C can be either global or local in extent. Global causes include (1) climatically-induced changes in the amount of terrestrial biomass which alter the average carbon isotopic composition of the oceanic reservoir (Shackleton, 1977), and (2) erosion and deposition of organic-rich, continental shelf sediments during sea level fluctuations which change the mean oceanic carbon: phosphorus ratio (Broecker, 1982 doi:10.1016/0079-6611(82)90007-6). Regional gradients of delta13C are created by remineralization of organic detritus within the deep ocean itself thus reflecting the distribution of water masses and modern thermohaline flow. Changes in a single geological record of benthic foraminiferal delta13C can result from any combination of these global and abyssal circulation effects. By sampling a large number of cores collected over a wide bathymetric range yet confined to a small geographical region we have minimized the ambiguity. We can assume that each delta13C record was equally affected by global causes of delta13C variation. The differences seen between the delta13C records must, therefore, reflect changes in the distribution of delta13C in the deep ocean. We interpret these differences in distribution in terms of changes in the ocean's abyssal circulation. Benthic foraminiferal carbon isotopic evidence from a suite of Sierra Leone Rise cores indicates that the deeper parts of the eastern Atlantic basins underwent a reduction in [O2] during the maximum of the last glaciation. Reduced advection of O2-rich deep water through low-latitude fracture zones, associated with increased delivery of organic matter to the deep ocean, lowered the delta13C of deep water SumCO2 at all depths below the sill separating the eastern and western Atlantic basins (Metcalf et al., 1964 doi:10.1016/0011-7471(64)91078-2). This decreased advection into the eastern Atlantic Ocean coincides with the overall decrease in deep water production in the North Atlantic during the last glacial maximum (Curry and Lohmann, 1982 doi:10.1016/0033-5894(82)90071-0; Boyle and Keigwin, 1982 doi:10.1126/science.218.4574.784; Schnitker, 1979 doi:10.1016/0377-8398(79)90020-3; Streeter and Shackleton, 1979 doi:10.1126/science.203.4376.168).

(Table S3) Benthic foraminiferal Mg/Ca, Li/Ca and stable isotope ratios of ODP Site 122-761

The sensitivities of benthic foraminiferal Mg/Ca and Li/Ca to bottom water temperature and carbonate saturation state have recently been assessed. Here we present a new approach that uses paired Mg/Ca and Li/Ca records to calculate simultaneous changes in temperature and saturation state. Using previously published records, we first use this approach to document a cooling of deep ocean waters associated with the establishment of the Antarctic ice sheet at the Eocene-Oligocene climate transition. We then apply this approach to new records of the Middle Miocene Climate Transition from ODP Site 761 to estimate variations in bottom water temperature and the oxygen isotopic composition of seawater. We estimate that the oxygen isotopic composition of seawater varied by ~1 per mil between the deglacial extreme of the Miocene Climatic Optimum and the glacial maximum following the Middle Miocene Climate Transition, indicating large amplitude variations in ice volume. However, the longer-term change between 15.3 and 12.5 Ma is marked by a ~1°C cooling of deep waters, and an increase in the oxygen isotopic composition of seawater of ~0.6 per mil. We find that bottom water saturation state increased in the lead up to the Middle Miocene Climate Transition and decreased shortly after. This supports decreasing pCO2 as a driver for global cooling and ice sheet expansion, in agreement with existing boron isotope and leaf stomatal index CO2 records but in contrast to the published alkenone CO2 records.

Carbonate concentrations of ODP Leg 115 samples

We measured carbonate concentrations in Pleistocene and Pliocene sediments deposited at Sites 709, 710, and 711. Carbonate concentrations exhibit low-amplitude, long-wave length (300-400 k.y. period) variations at the shallowest sites (709 and 710). Before 2.47 Ma, all three sites exhibit higher frequency (100 k.y. period) variations. The deepest site (711) exhibited low-amplitude variations and very low concentrations up to the Gauss/Matuyama magnetic reversal (2.47 Ma), then concentrations abruptly increased. After 2.47 Ma, carbonate concentrations at Site 711 exhibited the same periodic changes as at Site 709. Although a long wave-length periodicity (260-280 k.y.) occurs at these sites after 2.47 Ma, the 100 k.y. period is absent. The dominant periods observed in these data are those found in the eccentricity component of the earth's orbital geometry. Estimates of carbonate accumulation at Sites 709 and 710 document that surface-water productivity decreased near the Gauss/Matuyama magnetic reversal whereas accumulation at Site 711 increased. These results indicate that the rate of carbonate preservation in the deep Indian Ocean increased at that time. This increase in preservation may have re- sulted from a decrease in the production rate of carbonate in tropical oceans of the world. Carbonate accumulation esti- mated from sediments in shallow locations (~3000-3800 m) of the Atlantic and Pacific oceans also indicates that carbonate production decreased at this time. A consequence of lowered surface-water productivity is increased carbonate ion concentration of the deep ocean and better preservation of carbonate on the seafloor.

(Table 2) Integrated concentration, production and dissolution of silica in the water column of the southern Pacific

An intense diatom bloom developed within a strong meridional silicic acid gradient across the Antarctic Polar Front at 61°S, 170°W following stratification of the water column in late October/early November 1997. The region of high diatom biomass and the silicic acid gradient propogated southward across the Seasonal Ice Zone through time, with the maximum diatom biomass tracking the center of the silicic acid gradient. High diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of January 1998 (ca. 70 d) driving the gradient over 500 km to the south of its original position at the Polar Front. The bloom consumed 30 to >40 µM Si(OH)4 in the euphotic zone between about 60 and 66°S leaving near surface concentrations <2.5 µM and occasionally <1.0 µM in its wake. Integrated biogenic silica concentrations within the bloom averaged 410 mmol Si/m**2 (range 162-793 mmol Si/m**2). Average integrated silica production on two consecutive cruises in December 1997 and January 1998 that sampled the bloom while it was well developed were 27.5±6.9 and 22.6±20 mmol Si/m**2/d, respectively. Those levels of siliceous biomass and silica production are similar in magnitude to those reported for ice-edge diatom blooms in the Ross Sea, Antarctica, which is considered to be among the most productive regions in the Southern Ocean. Net silica production (production minus dissolution) in surface waters during the bloom was 16-21 mmol Si/m**2/d, which is sufficient for diatom growth to be the cause of the southward displacement of the silicic acid gradient. A strong seasonal change in silica dissolution : silica production rate ratios was observed. Integrated silica dissolution rates in the upper 100-150 m during the low biomass period before stratification averaged 64% of integrated production. During the bloom integrated dissolution rates averaged only 23% of integrated silica production, making 77% of the opal produced available for export to depth. The bloom ended in late January apparently due to a mixing event. Dissolution : production rate ratios increased to an average of 0.67 during that period indicating a return to a predominantly regenerative system. Our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around Antarctica also occur in the deep ocean near the Polar Front. The bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of Antarctica implying a role for such blooms in the formation of those sediments. Comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the Southern Ocean, which have been corrected for sediment focusing, indicate a burial efficiency of <=4.6% for biogenic silica. That efficiency is considerably lower than previous estimates for the Southern Ocean.

Geochemistry and minerals at DSDP Leg 81 Holes

The Leg 81 basalts, drilled either from the margins ("dipping reflectors" sequence: Holes 552, 553A, and 554A) or from the "continental" side (Hole 555) of the Rockall Plateau microcontinent, are strongly light rare-earth element (LREE) depleted oceanic tholeiites. The basalts from the four holes are almost similar. Most of their primary characteristics have been preserved, although they have suffered alteration by seawater. From the petrological and mineralogical points of view, they resemble deep-ocean-floor basalts but show some peculiarities (occurrence of pigeonite and ilmenite as normal components of the groundmass differentiation sequences toward ferrobasalts). Their geochemical characteristics are dominated by their extreme depletion in the most hygromagmaphile elements (Th, Ta, La, and Nb), the concentrations of which are sometimes lower than the corresponding chondritic values. Leg 81 basalts are thus clearly different from continental tholeiites (flood basalts): Possible equivalents in the Thulean Tertiary Magmatic Province include the LREE-depleted tholeiites from the Upper Basaltic Series of the Faeroe Islands and the Preshal Mhor basalt type from the British Tertiary Province.

Benthic foraminiferal Cd/Ca and isotope record from the South Atlantic

A depth transect of cores from 1268 to 3909 m water depth in the western South Atlantic are ideally situated to monitor the interocean exchange of deep water and variations in the relative strength of northern and southern sources of deep water production. Benthic foraminiferal Cd/Ca and d13C data suggest that Glacial North Atlantic Intermediate Water (GNAIW) extended at least as far south as 28°S in the western South Atlantic. The core of nutrient-depleted water was situated at ~1500 m, above and below water masses with higher nutrient concentrations. When examined in conjunction with published paired Cd/Ca and d13C from intermediate depth cores from other basins, it appears that the extent of GNAIW influence on the intermediate waters of the world's oceans was less than suggested previously. Differentiating among possible pathways for the glacial deep ocean (>3 km) requires a better understanding of the controls on Cd/Ca and d13C values of benthic foraminifera.

Stable carbon and oxygen isotope ratios of benthic foraminifera from Paleocene to Oligocene sediments

Benthic oxygen and carbon isotopic results from a depth transect on Maud Rise, Antarctica, provide the first evidence for Warm Saline Deep Water (WSDW) in the Paleogene oceans. Distinct reversals occur in the oxygen isotopic gradient between the shallower Hole 689B (Eocene depth ~1400 m; present-day depth 2080 m) and the deeper Hole 690B (Eocene depth ~2250 m; present-day depth 2914 m). The isotopic reversals, well developed by at least 46 Ma (middle middle Eocene), existed for much of the remaining Paleogene. We do not consider these reversals to be artifacts of differential diagenesis between the two sites or to have resulted from other potentially complicating factors. This being so, the results show that deep waters at Hole 690B were significantly warmer than deep waters at the shallower Hole 689B. A progressive decrease and eventual reversal in benthic to planktonic delta18O gradients in Hole 690B, demonstrate that the deeper waters became warmer relative to Antarctic surface waters during the Eocene. The warmer deep waters of the Paleogene are inferred to have been produced at middle to low latitudes, probably in the Tethyan region which contained extensive shallow-water platforms, ideal sites for the formation of high salinity water through evaporative processes. The ocean during the Eocene, and perhaps the Paleocene, is inferred to have been two-layered, consisting of warm, saline deep waters formed at low latitudes and overlain by cooler waters formed at high latitudes. This thermospheric ocean, dominated by halothermal circulation we name Proteus. The Neogene and modern psychrospheric ocean Oceanus is dominated by thermohaline circulation of deep waters largely formed at high latitudes. An intermediate condition existed during the Oligocene, with a three-layered ocean that consisted of cold, dense deep waters formed in the Antarctic (Proto-AABW), overlain by warm, saline deep waters from low latitudes, and in turn overlain by cool waters formed in the polar regions. This we name Proto-oceanus which combined both halothermal and thermohaline processes. The sequence of high latitude, major, climatic change inferred from the oxygen isotopic records is as follows: generally cooler earlier Paleocene; warming during the late Paleocene; climax of Cenozoic warmth during the early Eocene and continuing into the early middle Eocene; cooling mainly in a series of steps during the remainder of the Paleogene. Superimposed upon this Paleogene pattern, the Paleocene/Eocene boundary is marked by a brief but distinct warming that involved deep to surface waters and a reduction in surface to deep carbon and oxygen isotopic gradients. This event coincided with major extinctions among the deep-sea benthic foraminifers as shown by Thomas (1990 doi:10.2973/odp.proc.sr.113.123.1990). Salinity has played a major role in deep ocean circulation, and thus paleotemperatures cannot be inferred directly from the oxygen isotopic composition of Paleogene benthic foraminifers without first accounting for the salinity effect.

Calcite accumulation in the eastern equatorial Pacific

The eastern equatorial Pacific (EEP) is an important center of biological productivity, generating significant organic carbon and calcite fluxes to the deep ocean. We reconstructed paleocalcite flux for the past 30,000 years in four cores collected beneath the equatorial upwelling and the South Equatorial Current (SEC) by measuring ex230Th-normalized calcite accumulation rates corrected for dissolution with a newly developed proxy for "fraction of calcite preserved". This method produced very similar results at the four sites and revealed that the export flux of calcite was 30-50% lower during the LGM compared to the Holocene. The internal consistency of these results supports our interpretation, which is also in agreement with emerging data indicating lower glacial productivity in the EEP, possibly as a result of lower nutrient supply from the southern ocean via the Equatorial Undercurrent. However, these findings contradict previous interpretations based on mass accumulation rates (MAR) of biogenic material in the sediment of the EEP, which have been taken as reflecting higher glacial productivity due to stronger wind-driven upwelling.