David W. Murray

Pore Fluid Constraints on the Temperature and Oxygen Isotopic Composition of the Glacial Ocean

Pore fluids from the upper 60 meters of sediment 3000 meters below the surface of the tropical Atlantic indicate that the oxygen isotopic composition (δ18O) of seawater at this site during the last glacial maximum was 0.8 ± 0.1 per mil higher than it is today. Combined with the δ18O change in benthic foraminifera from this region, the elevated ratio indicates that the temperature of deep water in the tropical Atlantic Ocean was 4°C colder during the last glacial maximum. Extrapolation from this site to a global average suggests that the ice volume contribution to the change in δ18O of foraminifera is 1.0 per mil, which partially reconciles the foraminiferal oxygen isotope record of tropical sea surface temperatures with estimates from Barbados corals and terrestrial climate proxies.

The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2.

Most global biogeochemical processes are known to respond to climate change, some of which have the capacity to produce feedbacks through the regulation of atmospheric greenhouse gases. Marine denitrification—the reduction of nitrate to gaseous nitrogen—is an important process in this regard, affecting greenhouse gas concentrations directly through the incidental production of nitrous oxide, and indirectly through modification of the marine nitrogen inventory and hence the biological pump for CO2. Although denitrification has been shown to vary with glacial–interglacial cycles, its response to more rapid climate change has not yet been well characterized. Here we present nitrogen isotope ratio, nitrogen content and chlorin abundance data from sediment cores with high accumulation rates on the Oman continental margin that reveal substantial millennial-scale variability in Arabian Sea denitrification and productivity during the last glacial period. The detailed correspondence of these changes with Dansgaard–Oeschger events recorded in Greenland ice cores indicates rapid, century-scale reorganization of the Arabian Sea ecosystem in response to climate excursions, mediated through the intensity of summer monsoonal upwelling. Considering the several-thousand-year residence time of fixed nitrogen in the ocean, the response of global marine productivity to changes in denitrification would have occurred at lower frequency and appears to be related to climatic and atmospheric CO2 oscillations observed in Antarctic ice cores between 20 and 60 kyr ago.

Nonstationary Phase of the Plio-Pleistocene Asian Monsoon

Paleoclimate records indicate that the strength of the Asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods (41,000 and 23,000 years, respectively) and the extent of Northern Hemisphere glaciation. Over the past 2.6 million years, the timing (phase) of strong monsoons has changed by ∼83 degrees in the precession and ∼124 degrees in the obliquity bands relative to the phase of maximum global ice volume (inferred from the marine oxygen isotope record). These results suggest that one or both of these systems is nonstationary relative to orbital forcing.

Fifty thousand years of Arctic vegetation and megafaunal diet

Although it is generally agreed that the Arctic flora is among the youngest and least diverse on Earth, the processes that shaped it are poorly understood. Here we present 50 thousand years (kyr) of Arctic vegetation history, derived from the first large-scale ancient DNA metabarcoding study of circumpolar plant diversity. For this interval we also explore nematode diversity as a proxy for modelling vegetation cover and soil quality, and diets of herbivorous megafaunal mammals, many of which became extinct around 10 kyr bp (before present). For much of the period investigated, Arctic vegetation consisted of dry steppe-tundra dominated by forbs (non-graminoid herbaceous vascular plants). During the Last Glacial Maximum (25–15 kyr bp), diversity declined markedly, although forbs remained dominant. Much changed after 10 kyr bp, with the appearance of moist tundra dominated by woody plants and graminoids. Our analyses indicate that both graminoids and forbs would have featured in megafaunal diets. As such, our findings question the predominance of a Late Quaternary graminoid-dominated Arctic mammoth steppe.

Climatically linked oscillations in Arabian Sea denitrification over the past 1 m.y.: Implications for the marine N cycle

Water column and core-top δ15N data show that Arabian Sea denitrification produces large nitrogen isotopic enrichments that are regionally recorded with fidelity in the sediments. These results facilitate interpretation of a 1 m.y. δ15N record for Ocean Drilling Program site 722B on the Owen Ridge in terms of climatically linked oscillations in denitrification at the major orbital periods. As at present, denitrification was greatest during interglacial periods and, apparently, was not active during most glacial intervals. Cross-spectral analysis of δ15N with foraminiferal δ18O (global climate/sea level index) and lithogenic grain size (monsoon strength index) suggests forcings by changes in hydrography and productivity acting through the extent and intensity of the oxygen minimum zone. The data suggest that denitrification may be an internal forcing mechanism for climate change during major glacial/interglacial transitions through influence on marine N inventory and atmospheric CO2. However, compensation or amplification may occur from other sinks (sediment denitrification) or sources (nitrogen fixation).

Evidence for resuspension of rebound particles from near-bottom sediment traps

Abstract Near-bottom sediment trap moorings were recovered at three sites in the North Equatorial Pacific. Total particulate fluxes recorded within 30–350 m of the ocean floor were greater than those recorded in the mid-water column. A simple two-component mixing model using the most organic-rich surface sediments sampled and an extrapolated mid-water column flux does not account for increases in the major biogenic components (organic carbon, calcium carbonate and opal). The resuspension of rebound particles (those particles) that have settled through the water column but have not become incorporated into the sediments) may account for the observed flux near the ocean floor.

Late Quaternary climate change from δ18O records of multiple species of planktonic foraminifera: High‐resolution records from the Anoxic Cariaco Basin, Venezuela

Seasonal trade wind-induced upwelling along the southern margin of the Caribbean Sea occurs in response to the annual migration of the Intertropical Convergence Zone. Laminated, high deposition rate sediments of the Cariaco Basin, a small anoxic basin on the Venezuelan continental shelf, clearly record large changes in the past intensity of this upwelling. Because sediments of the Cariaco Basin are largely unbioturbated, they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate. Oxygen isotope data (δ18O) from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr. The δ18O data from Globigerina bulloides, after correction for nonequilibrium precipitation, are used as a monitor of sea surface conditions during the winter-spring upwelling season. The δ18O data from white Globigerinoides ruber are used as a measure of annual-average conditions in the near surface, while pink G. ruber data are consistent with use as an index of endmember conditions during the summer-fall nonupwelling season. Data from the deeper dwelling Neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline. During the last glacial, δ18O data from G. ruber and generally reduced interspecific differences indicate cooling of surface waters over the Cariaco Basin by up to 4°C. This longer-term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger Caribbean. Superimposed on this pattern, between 12.6 and ∼10 ka, is a convergence of δ18O data between G. bulloides and N. dutertrei, implying much stronger upwelling during the last deglaciation. This scenario is consistent with other evidence for high productivity at this time. At ∼14 ka, a sharp δ18O depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern Caribbean, suggesting either higher regional rainfall or the influence of glacial melting in the Andes. Minimum δ18O values of pink G. ruber around 6–7 ka record warmer summer sea surface temperatures and/or decreased salinity in the mid-Holocene.

Timescale and paleoceanographic implications of a 3.6 m.y. oxygen isotope record from the northeast Indian Ocean (Ocean Drilling Program Site 758)

Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100-, 41- and 23-kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.

Al-to-oxide and Ti-to-organic linkages in biogenic sediment: relationships to paleo-export production and bulk Al/Ti

Abstract To increase our understanding of the mechanisms that control the distribution of Al and Ti within marine sediment, we performed sequential extractions targeting the chemical signatures of the loosely bound, exchangeable, carbonate, oxide, organic, opal, and residual fraction of sediment from a carbonate-dominated regime (equatorial Pacific) and from a mixed opal–terrigenous regime (West Antarctic Peninsula). We observe a systematic partitioning of Al and Ti between sediment phases that is related to bulk Al/Ti. We show that, where we can quantify an Alexcess component, the dissolved Al is preferentially affiliated with the oxide fraction, resulting in Al/Ti molar ratios of 500–3000. This is interpreted as the result of surface complexation in the water column of dissolved Al onto oxyhydroxides. We also observe a previously undetected Tiexcess with as much as 80% of the total Ti in the organic fraction, which is most likely a function of metal-organic colloidal removal from the water column. In samples where the excess metals are obscured by the detrital load, the Al and Ti are almost exclusively found in the residual phase. This argues for the paired removal of Al (preferentially by the oxide component) and Ti (preferentially by the organic component) from the water column by settling particulate matter. This research builds upon earlier work that shows changes in the bulk ratio of Al to Ti in carbonate sediment from the central-equatorial Pacific that coincide with changes in the sedimentary bulk accumulation rate (BAR). The ratios that are observed are as much as three times higher than typical shale values, and were interpreted as the result of scavenging of dissolved Al onto particles settling in the water column. Because this non-terrigenous Alexcess accounts for up to 50% of the total sedimentary Al inventory and correlates best with BAR, the bulk Al/Ti may be a sensitive tracer of particle flux and, therefore, export production. Because we show that the excess metals are the result of scavenging processes, the bulk Al/Ti may be considered a sensitive proxy for this region.

Hypoxia in the Upper Half of Narragansett Bay, RI, During August 2001 and 2002

Abstract Narragansett Bay, RI, is considered to be a relatively well-mixed estuary not subject to extensive seasonal stratification and hypoxia. However, results of surveys of dissolved oxygen (DO) for the upper half of Narragansett Bay on August 15, 2001 and on August 6, 2002 have documented evidence of wide-area intermittent subpycnoclinal hypoxia (≤ 3 mg l−1). For the August 2001 survey, severe hypoxic to near-anoxic levels were confined to the Providence River, the western side of Greenwich Bay, and a small area of Mount Hope Bay, but hypoxic levels below 2 mg l−1 were also experienced on the western side of the Upper Bay in an extensive, shallow oxygen minimum. Hypoxic bottom waters (≤ 3 mg l−1) extended from the Upper Bay into the upper West Passage. Hypoxic waters covered approximately 66 km2 (36%) of the survey area for August 15, 2001. A more extensive and severe hypoxic event occurred during the August 2002 survey, when near-bottom waters of the entire Providence River and a large area of the Upper Bay and upper East Passage were severely hypoxic to near-anoxic, while other parts of the Upper Bay, upper East Passage and upper West Passage were hypoxic at depths greater than 5 m. Limited data for Mount Hope Bay in August 2002 documented small hypoxic areas of the southern end of that subembayment. The total hypoxic area for August 6, 2002 was approximately 93 km2 (65%) of the total area surveyed. Decreased estuarine circulation due to a severe drought may have contributed to the wider extent of hypoxic and near-anoxic waters in large areas of the upper half of Narragansett Bay recorded in the August 6, 2002 survey as compared with the August 15, 2001 survey. Results of the oxygen surveys affirm sediment profile camera work and limited benthic studies that previously suggested parts of the Mid Bay have become subject to increased organic loading impacts. These impacts can take place even under drought conditions, when only point source nutrients are the major contributors to nutrient loadings entering the upper half of Narragansett Bay.