Stable isotopic record of late Pliocene and Pleistocene foraminifera of ODP Site 114-704

We studied the stable isotopic and carbonate stratigraphy of ODP Hole 704A to reconstruct the paleoceanographic evolution of the eastern subantarctic sector of the South Atlantic Ocean. Site 704 is well positioned with respect to latitude (46°52.8'S, 7°25.3'E) and bathymetry (2532 m) to monitor past migrations in the position of Polar Front Zone (PFZ) and changes in deep-water circulation during the late Pliocene-Pleistocene. Several important changes occurred in proxy paleoceanographic indicators across the Gauss/Matuyama boundary at 2.47 Ma: (1) accumulation rates of biogenic sedimentary components increased by an order of magnitude (Froelich et al., this volume); (2) planktonic d1 8O values increased by an average of 0.5 per mil; (3) the amplitude of the benthic d18O signal increased; (4) the accumulation rate of ice-rafted detritus increased several fold (Warnke and Allen, this volume); and (5) carbon isotopic ratios of benthic foraminifers decreased by 0.5 per mil, as did the d13C of the fine-fraction carbonate by 1.5 per mil (Mead et al., 1991, doi:10.2973/odp.proc.sr.114.152.1991), but no change occurred in planktonic foraminiferal d13C values. Most of these changes are consistent with more frequent expansions and contractions of the PFZ over Site 704 after 2.47 Ma, bringing cold, nutrient-rich waters to 47°S that stimulated both carbonate and siliceous productivity. The synchronous increase in d18O values and ice-rafted detritus accumulation in Hole 704A indicates that the 2.4 Ma paleoceanographic event included ice volume growth on both Antarctica and Northern Hemisphere continents. The decrease in benthic d13C values indicates that the ventilation rate of Southern Ocean deep water decreased and the nutrient content increased during glacial events after 2.5 Ma. At the Gauss/Matuyama boundary, benthic d13C values of the Southern Ocean shifted toward those of the Pacific end member, indicating a decrease in the relative mixing ratio of Northern Component Water and Circumpolar Deep Water. During the early Matuyama (~2.3 to 1.7 Ma), the PFZ generally occupied a southerly position with respect to Site 704 and carbonate productivity prevailed. Exceptions to these general conditions occurred during strong glacial events of the early Matuyama (e.g., isotopic stages 82, 78, 74, and 70), when the PFZ migrated to the north and opal sedimentation predominated at Site 704. At 1.7 Ma, the PFZ migrated toward the equator and occupied a more northerly position for a prolonged interval between ~1.7 and 1.5 Ma. Beginning at ~1.5-1.4 Ma, surface and bottom water parameters (d18O, d13C, %CaCO3, and %opal) in the subantarctic South Atlantic became highly correlated such that glacial events (d18O maxima) corresponded to d13C and carbonate minima and opal maxima. This pattern is typical of the correlation found during the latest Pleistocene in the Southern Ocean (Charles and Fairbanks, in press). This event coincided with increased suppression of Northern Component Water during glacial events after 1.5 Ma (Raymo et al., 1990, doi:10.1016/0012-821X(90)90051-X), which may have influenced the climatology of the Southern Hemisphere by altering the flux of heat and salt to the Southern Ocean).

Seawater carbonate chemistry, gross photosynthesis and metabolically induced rate of pH change during experiments with macroalgae, 2012

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 increase; CO32 decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 uptake, and so HCO3-using macroalgae may benefit in future seawater with elevated CO2.

Clay mineralogical assemblages, Sm-Nd concentrations, Sm and Nd isotopic ratios and oxygen isotope ratios, OIS number and the age model of ODP Site 105-646

We present 40 Sm-Nd isotope measurements of the clay-size (<2 µm) fractions of sediments from the Southern Greenland rise (ODP-646) that span the last 365 kyr. These data track changes in the relative supply of fine particles carried into the deep Labrador Sea by the Western Boundary Under Current (WBUC) back to the fourth glacial-interglacial cycles. Earlier studies revealed three general sources of particles to the core site: (i) Precambrian crustal material from Canada, Greenland, and/or Scandinavia (North American Shield - NAS), (ii) Palaeozoic or younger crustal material from East Greenland, NW Europe, and/or western Scandinavia (Young Crust - YC) and (iii) volcanic material from Iceland and the Mid-Atlantic Ridge (MAR). Clay-size fractions from glacial sediments have the lowest Nd isotopic ratios. Supplies of young crustal particles were similar during glacial oxygen isotope stages (OIS) 2, 6, and 10. In contrast the mean volcanic contributions decreased relative to old craton material from OIS 10 to OIS 6 and then from OIS 6 to OIS 2. The glacial OIS 8 interval displays a mean Sm/Nd ratio similar to those of interglacials OIS 1, 5, and 9. Compared with other interglacials, OIS 7 was marked by a higher YC contribution but a similar ~30% MAR supply. The overall NAS contribution dropped by a factor of 2 during each glacial/interglacial transition, with the MAR contribution broadly replacing it during interglacials. To decipher between higher supplies and/or dilution, particle fluxes from each end member were estimated. Glacial NAS fluxes were systematically higher than interglacial fluxes. During the time interval examined, fine particle supplies to the Labrador Sea were strongly controlled by proximal ice-margin erosion and thus echoed the glacial stage intensity. In contrast, the WBUC-carried MAR supplies from the eastern basins did not change significantly throughout the last 365 kyr, except for a marked increase in surface-sediments that suggests unique modern conditions. Distal WBUC-controlled inputs from the Northern and NE North Atlantic seem to have been less variable than proximal supplies linked with glacial erosion rate.

Abrupt vegetation change after the Late Quaternary megafaunal extinction in southeastern Australia

A substantial extinction of megafauna occurred in Australia between 50 and 45 kyr ago, a period that coincides with human colonization of Australia. Large shifts in vegetation also occurred around this time, but it is unclear whether the vegetation changes were driven by the human use of fire-and thus contributed to the extinction event-or were a consequence of the loss of megafaunal grazers. Here we reconstruct past vegetation changes in southeastern Australia using the stable carbon isotopic composition of higher plant wax n-alkanes and levels of biomass burning from the accumulation rates of the biomarker levoglucosan from a well-dated sediment core offshore from the Murray-Darling Basin. We find that from 58 to 44 kyr ago, the abundance of plants with the C-4 carbon fixation pathway was generally high-between 60 and 70%. By 43 kyr ago, the abundance of C-4 plants dropped to 30% and biomass burning increased. This transient shift lasted for about 3,000 years and came after the period of human arrival and directly followed megafauna extinction at 48.9-43.6 kyr ago. We conclude that the vegetation shift was not the cause of the megafaunal extinction in this region. Instead, our data are consistent with the hypothesis that vegetation change was the consequence of the extinction of large browsers and led to the build-up of fire-prone vegetation in the Australian landscape.

Isotopic composition and chemistry of pore waters from ODP Site 195-1201 sediments, West Philippine Basin

This report summarizes chemical and isotopic data from Ocean Drilling Program Leg 195 Site 1201. Pore water is divided into three intervals based on the rate of chemical change with depth. The shallowest interval is the red clay unit between 1.26 and 56.40 meters below seafloor (mbsf). In this section, there are overall decreases in the concentrations of alkalinity, boron, lithium, magnesium, potassium, sodium, and sulfate, whereas concentrations of calcium and chloride increase. Values of d18O and dD plot near standard mean ocean water to the right of the global meteoric water line (GMWL). Five samples from 72.60 and 83.33 mbsf yielded pore water for analyses. These samples help define a trend in the second interval, which is between 56.4 and 238.98 mbsf. Here, concentrations of magnesium, potassium, sodium, and sulfate decease, whereas concentrations of boron, calcium, and chloride increase. Concentrations of alkalinity and lithium remain roughly constant. The deepest interval, between 238.04 and 504.8 mbsf, has comparatively slower decreases of sodium and sulfate, increases of calcium and chloride, slow increases of alkalinity and lithium, and roughly constant concentrations of magnesium, potassium, and boron. Values of d18O and dD in pore water between 146.98 and 504.80 mbsf plot in a linear trend to the right of the GMWL.

Radiocarbon ages, dose rates, water isotopic composition and hydrochemistry of samples from Komakuk Beach and Herschel Island

Terrestrial permafrost archives along the Yukon Coastal Plain (northwest Canada) have recorded landscape development and environmental change since the Late Wisconsinan at the interface of unglaciated Beringia (i.e. Komakuk Beach) and the northwestern limit of the Laurentide Ice Sheet (i.e. Herschel Island). The objective of this paper is to compare the late glacial and Holocene landscape development on both sides of the former ice margin based on permafrost sequences and ground ice. Analyses at these sites involved a multi-proxy approach including: sedimentology, cryostratigraphy, palaeoecology of ostracods, stable water isotopes in ground ice, hydrochemistry, and AMS radiocarbon and infrared stimulated luminescence (IRSL) dating. AMS and IRSL age determinations yielded full glacial ages at Komakuk Beach that is the northeastern limit of ice-free Beringia. Herschel Island to the east marks the Late Wisconsinan limit of the northwest Laurentide Ice Sheet and is composed of ice-thrust sediments containing plant detritus as young as 16.2 cal ka BP that might provide a maximum age on ice arrival. Late Wisconsinan ice wedges with sediment-rich fillings on Herschel Island are depleted in heavy oxygen isotopes (mean d18O of -29.1 per mil); this, together with low d-excess values, indicates colder-than-modern winter temperatures and probably reduced snow depths. Grain-size distribution and fossil ostracod assemblages indicate that deglaciation of the Herschel Island ice-thrust moraine was accompanied by alluvial, proluvial, and eolian sedimentation on the adjacent unglaciated Yukon Coastal Plain until ~11 cal ka BP during a period of low glacio-eustatic sea level. The late glacial-Holocene transition was marked by higher-than-modern summer temperatures leading to permafrost degradation that began no later than 11.2 cal ka BP and caused a regional thaw unconformity. Cryostructures and ice wedges were truncated while organic matter was incorporated and soluble ions were leached in the thaw zone. Thermokarst activity led to the formation of ice-wedge casts and deposition of thermokarst lake sediments. These were subsequently covered by rapidly accumulating peat during the early Holocene Thermal Maximum. A rising permafrost table, reduced peat accumulation, and extensive ice-wedge growth resulted from climate cooling starting in the middle Holocene until the late 20th century. The reconstruction of palaeolandscape dynamics on the Yukon Coastal Plain and the eastern Beringian edge contributes to unraveling the linkages between ice sheet, ocean, and permafrost that have existed since the Late Wisconsinan.

Isotopic ratios and concentrations for mineral-associated and particulate organic fractions of sediment core Wilson_Lake samples

The flux of sediment and organic carbon from continents to the coastal ocean is an important factor governing organic burial in coastal sediments, and these systems preserve important records of environmental and biogeochemical conditions during past global change events. Burial of organic materials in coastal systems can be promoted by chemical resilience or through protection by association with mineral surfaces, but the role and influence of these processes on organic records from ancient sediments is poorly known. We studied sediment and organic matter burial as particulate organic matter (POM) and mineral-bound organic matter (MOM) in near-shore marine sediments from the Wilson Lake core (New Jersey, USA) that span the Paleocene-Eocene thermal maximum (PETM), a climatic perturbation 55.9 Myr ago. Our results show that distinct POM and MOM fractions can be isolated from sediments. Both fractions appear to be dominated by terrestrial material, but POM consisted primarily of recently synthesized material whereas MOM included a significant fraction of pre-aged organic matter from soils or ancient sediments. Variation in organic burial through the PETM is associated with changes in inorganic nitrogen burial, clay mineralogy, and clastic grain size that we associate with enhanced continental weathering, erosion and redeposition of ancient kaolinites, and eustatic sea level variation, respectively. These results provide a new perspective on factors governing carbon burial and carbon isotope records in ancient marine margin settings and offer information on rate and phasing of late Paleocene/early Eocene Earth system changes that may constrain interpretations of the cause of the PETM climate change event.

Stable isotopic composition and carbonate concentrations of sediments of the Cretaceous/Paleogene boundary in the Antarctic

Stable isotopic records across the Cretaceous/Paleogene (K/P) boundary in Maud Rise Holes 689B and 690C indicate that significant climatic changes occurred during the latest Cretaceous, beginning approximately 500 k.y. prior to the mass extinction event and the enrichment of iridium at the K/P boundary (66.4 Ma). An oxygen isotopic decrease of ~0.7 per mil - ~1.0 per mil is recorded in the Late Cretaceous planktonic and benthic foraminifers between 66.9 and 66.6 Ma. The negative isotope excursion was followed by a positive excursion of similar magnitude between 66.6 Ma (latest Cretaceous) and ~66.3 Ma (earliest Paleocene). No other isotopic excursions of this magnitude are recorded in the planktonic and benthic microfossil records 1.0 m.y prior to, and for 2.0 m.y following the mass extinction event at the K/P boundary. The magnitude and duration of these isotopic excursions were similar to those at the Paleocene/Eocene and Eocene/Oligocene boundaries. A major d13C excursion occurred 200 k.y. prior to the boundary, involving a positive shift in planktonic and benthic d13C of ~0.5 per mil - 0.75 per mil. Similar changes observed in other deep-sea sequences indicate that this reflected a global change in d13C of the oceanic total dissolved carbon (TDC) reservoir. The magnitude of this inferred carbon reservoir change and its association with high latitude surface-water temperature changes recorded in the d18O records implies that it was linked to global climate change through feedback loops in the carbon cycle. At the K/P boundary, the surface-to-deep water d13C gradient is reduced by approximately 0.6 per mil - ~0.2 per mil. However, unlike sequences elsewhere, the planktonic-benthic d13C gradient (Delta d13C) was not eliminated in the Antarctic. The surface-to-deep water gradient was re-established gradually during the 400 k.y. following the mass extinction. Full recovery of the Delta d13C occurred by ~60.0 Ma. In addition to the reduced vertical d13C gradient across the K/P boundary, there was a negative excursion in both planktonic and benthic d13C beginning approximately 100 k.y. after the boundary (66.3 Ma). This excursion resulted in benthic d13C values in the early Paleogene that were similar to those in the pre-K/P boundary intervals. This negative shift appears to reflect a change in the d13C of the oceanic TDC reservoir shift that may have resulted from reduced carbon burial and/or increased carbon flux to the oceans. Any model that attempts to explain the demise of the oceanic plankton at the end of the Cretaceous should consider the oceanic environmental changes that were occurring prior to the massive extinction event.

Age model and sedimentation rate of core M174/KL11 (Table 1)

Stable isotope and faunal records from the central Red Sea show high-amplitude oscillations for the past 380,000 years. Positive delta18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53 per mil and 55 per mil are inferred from pteropod and benthic foraminifera delta18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic delta18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically - driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.