Julian P. Sachs

The geochemical cycle of rhenium: a reconnaissance

Rhenium (Re) is one of a suite of elements (including uranium and molybdenum) that display conservative behavior in seawater and are enriched in anoxic sediments. The decay of187Re to187Os provides a geochronometer in ancient sedimentary rocks and gives rise to Os-isotopic variations in nature. In order to better characterize its sources to seawater, Re was measured in three major rivers (Amazon, Orinoco, Ganges-Brahmaputra) and some of their tributaries. Re concentrations span four orders of magnitude (from < 0.02 to 400 pmol/kg), with the highest concentrations found in rivers draining black shales in the Venezuelan Andes. Mainstream Re levels in the three rivers are between 1 and 10 pmol/kg, with a flux weighted average of 2.3 pmol/kg. The residence time for Re in the oceans is estimated to be 750,000 yr with respect to river inputs. Re profiles from the Atlantic and Pacific Oceans confirm that Re behaves conservatively in seawater, with no significant uptake onto particles and/or recycling within the water column. This is also true in the anoxic water column of the Black Sea. Re removal into anoxic sediments occurs at or below the sediment water interface, as demonstrated in sediment pore waters from Chesapeake Bay. In oxic sediments, Re is not cycled with manganese oxides, and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component, or in manganese nodules. Burial of Re in anoxic sediments, which accumulate on 0.3% of the ocean floor, removes approximately 50% of the riverine Re flux to the oceans. Hence, oceanic Re concentrations may be very sensitive to changes in the area of anoxic sedimentation.

Stable isotope constraints on the nitrogen cycle of the Mediterranean Sea water column

We usedthe nitrogen isotope ratio of algae, suspend edparticles andnitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in 15 Ni n the eastern basin (� 0.370.5%) than in the western basin (+2.471.4%), suggesting that nitrogen suppliedby biological N 2 fixation may be an important source of new nitrogen in the eastern basin, where preformednitrate from the Atlantic Ocean could have been depleted during its transit eastward. The d 15 N of nitrate in the deep Mediterranean (B3% in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5%), also suggesting an important role for N2 fixation. The eastwardd ecrease in the d 15 No f surface PON is greater than the eastwarddecrease in the d 15 N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the d 15 N of phytoplankton biomass from the d 15 N of the nitrate upwelledfrom below. Variations in prod uctivity associatedwith frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the d 15 N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) andnitrate concentration in the Alboran Sea. 15 N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.470.7%. The d 15 N in sinking particles caught at 100 m depth (4.9–5.6%) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Dd 15 N[PON-chlorin]=+6.471.4%) in the surface, similar to the offset reportedearlier in cultures for cellular N andchl- a. This indicates that d 15 N of phytoplankton biomass was retainedin surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity. r 2002 Elsevier Science Ltd. All rights reserved.

Eastern tropical Pacific hydrologic changes during the past 27,000 years from D/H ratios in alkenones

[1] The tropical Pacific plays a central role in the climate system by providing large diabatic heating that drives the global atmospheric circulation. Quantifying the role of the tropics in late Pleistocene climate change has been hampered by the paucity of paleoclimate records from this region and the lack of realistic transient climate model simulations covering this period. Here we present records of hydrogen isotope ratios (dD) of alkenones from the Panama Basin off the Colombian coast that document hydrologic changes in equatorial South America and the eastern tropical Pacific over the past 27,000 years (a) and the past 3 centuries in detail. Comparison of alkenone dD values with instrumental records of precipitation over the past � 100 a suggests that dD can be used as a hydrologic proxy. On long timescales our records indicate reduced rainfall during the last glacial period that can be explained by a southward shift of the mean position of the Intertropical Convergence Zone and an associated reduction of Pacific moisture transport into Colombia. Precipitation increases at � 17 ka in concert with sea surface temperature (SST) cooling in the North Atlantic and the eastern tropical Pacific. A regional coupled model, forced by negative SST anomalies in the Caribbean, simulates an intensification of northeasterly trade winds across Central America, increased evaporative cooling, and a band of increased rainfall in the northeastern tropical Pacific. These results are consistent with the alkenone SST and dD reconstructions that suggest increasing precipitation and SST cooling at the time of Heinrich event 1.

Increased productivity in the subantarctic ocean during Heinrich events

Massive iceberg discharges from the Northern Hemisphere ice sheets, ‘Heinrich events’, coincided with the coldest periods of the last ice age. There is widespread evidence for Heinrich events and their profound impact on the climate and circulation of the North Atlantic Ocean, but their influence beyond that region remains uncertain. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to document eight periods of increased productivity in the subpolar Southern Ocean during the past 70,000 years that occurred within 1,000–2,000 years of a Northern Hemisphere Heinrich event. We discuss possible causes for such a link, including increased supply of iron from upwelling and increased stratification during the growing season, which imply an alteration of the global ocean circulation during Heinrich events. The mechanisms linking North Atlantic iceberg discharges with subantarctic productivity remain unclear at this point. We suggest that understanding how the Southern Ocean was altered during these extreme climate perturbations is critical to understanding the role of the ocean in climate change.

Nitrogen and carbon isotopic ratios of chlorophyll from marine phytoplankton

The relationship between the nitrogen and carbon isotopic ratios of chlorophyll a and total biomass was explored in cultured marine phytoplankton to assess the utility of chlorophyll as an isotopic proxy for photoautotrophs. A near constant nitrogen isotopic depletion of 5.06 6 1.13‰ (95% confidence interval) in chlorophyll a relative to total nitrogen was observed in 8 species. This value was similar to isotopic differences between chlorophyll a and marine particles (5.27 6 1.48‰ (1 s); n 5 6) and sediments (5.39 6 0.67‰ (1s); n 5 4) in a variety of settings. These findings suggest that a 5.1‰ isotopic depletion of chlorophyll a relative to total algal nitrogen is a robust relationship that justifies the use of chlorophyll as a nitrogen isotopic surrogate for photoautotrophs. Although interspecies differences in Dd 15 Ncell-Chla exist, and growth rate has a small effect on this parameter, the field data suggest these factors are probably minimized in the ocean where multiple species and growth rates occur. The nitrogen isotopic depletion of chlorophyll a probably occurs during the transamination of glutamic acid in d-aminolevulinic acid biosynthesis, the first committed precursor to chlorophyll. The carbon isotopic composition of chlorophyll from 12 batch cultures and 7 species of marine phyto- plankton was elevated by 0.32 6 1.61‰ (95% confidence interval) relative to total cellular carbon. No significant interspecies variance was observed that was not attributable to intraspecies variance. There was a moderate inverse correlation between growth rate and Dd 13 Ccell-Chla, and it is hypothesized that this parameter is largely responsible for the large range of intraspecies Dd 13 Ccell-Chla values observed in batch cultures. Copyright

Holocene history of ENSO variance and asymmetry in the eastern tropical Pacific

El Niño shifted between the center and the East El Niño has changed quite a bit over the past 10,000 years. During some periods it was less variable than now, and during others it shifted from its current locale toward the central Pacific. Carré et al. analyzed the shells of mollusks from Peru to construct a record of the El Niño–Southern Oscillation (ENSO) in the eastern Pacific over the Holocene period. They compared this record with other records from the rest of the Pacific to reveal how much the strength and frequency of El Niños changed and how their positions varied. Science, this issue p. 1045 El Niño–Southern Oscillation warming of the sea surface was strongly skewed toward the central Pacific in the Mid-Holocene. Understanding the response of the El Niño–Southern Oscillation (ENSO) to global warming requires quantitative data on ENSO under different climate regimes. Here, we present a reconstruction of ENSO in the eastern tropical Pacific spanning the past 10,000 years derived from oxygen isotopes in fossil mollusk shells from Peru. We found that ENSO variance was close to the modern level in the early Holocene and severely damped ~4000 to 5000 years ago. In addition, ENSO variability was skewed toward cold events along coastal Peru 6700 to 7500 years ago owing to a shift of warm anomalies toward the Central Pacific. The modern ENSO regime was established ~3000 to 4500 years ago. We conclude that ENSO was sensitive to changes in climate boundary conditions during the Holocene, including but not limited to insolation.

Northern timing of deglaciation in the eastern equatorial Pacific from alkenone paleothermometry

(1) The equatorial cold tongue (ECT) of the eastern Pacific is the most dynamic ocean region in the worlds tropics and sets the tempo for global climate anomalies arising from El Nino-Southern Oscillation (ENSO) events. This regions deglaciation history and relationship with north and south polar climates remains poorly understood, impeding integration of tropical Pacific ocean-atmosphere dynamics and ENSO variability in our understanding of glacial cycles. Here we present alkenone reconstructions of sea surface temperature (SST) across the last glacial termination from five ECT cores east of the Galapagos Islands. A composite index of SST based on these demonstrates strong temporal affinity with the two-step deglaciation of the Northern Hemisphere, composed of two distinct warming steps at the beginning of the Bolling and the end of the Younger Dryas intervals. Within dating uncertainty, warming in the ECT began in phase with the Bolling excursion, was followed by a 2-3 ka plateau, and resumed with a second pulse at the end of the Younger Dryas. On the basis of our reconstructions, about two thirds of the warming materialized at or after the end of the Younger Dryas, implying a marked delay in the regions response to deglaciation. The results challenge the prevailing paradigm that ECT deglacial history conforms to an Antarctic timing, commonly attributed to advection from the Southern Ocean through an interior oceanic link, or to synchronous response of both regions to CO2 forcing. Our results emphasize instead the role of dynamical adjustments linked to Northern Hemisphere processes, most likely transmitted through the atmosphere.

Relation of subtropical Atlantic temperature, high-latitude ice rafting, deep water formation, and European climate 130,000–60,000 years ago

Abstract A new, high-resolution record of sea surface temperature from the subtropical western North Atlantic documents a series of abrupt coolings within marine isotope stage 5 which can be objectively correlated with marine-core evidence for increased ice-sheet discharge at subpolar latitudes and reduced North Atlantic deep water formation. These results indicate that ice-sheet forcing of deep ocean circulation influenced surface temperatures over much of the North Atlantic. A proposed correlation to the pollen record of Grande Pile, France, indicates that each cold event seen in the pollen sequence has a unique counterpart in the record of subtropical ocean temperature. If this correlation is correct, it suggests that the warmest part of the European Eemian ended suddenly in response to oceanographic changes, and that the subsequent post-temperate phase of the Eemian extended well into the interval of ice sheet growth corresponding to marine isotope substage 5d.

A late-glacial high-resolution site and source temperature record derived from the EPICA Dome C isotope records (East Antarctica)

The timing and synchronisation of Greenland and Antarctic climate events that occurred during the last glacial period are still under debate, as is the magnitude of temperature change associated with these events. Here we present detailed records of local and moisture-source temperature changes spanning the period 27-45 kyr BP from water stable isotope measurements (deltaD and delta(18)O) in the recently drilled EPICA Dome C ice core, East Antarctic plateau. Using a simple isotopic model, site (DeltaT(site)) and source (DeltaT(source)) temperatures are extracted from the initial 50-yr high-resolution isotopic records, taking into account the changes in seawater isotopic composition. The deuterium isotope variability is very similar to the less precise deltaD record from the Vostok ice core, and the site temperature inversion leads to a temperature profile similar to the classical palaeothermometry method, due to compensations between source and ocean water corrections. The reconstructed DeltaT(site) and DeltaT(source) profiles show different trends during the glacial: the former shows a decreasing trend from the warm Al event (38 kyr BP) toward the Last Glacial Maximum, while the latter shows increasing values from 41 to 28 kyr BP. The low-frequency deuterium excess fluctuations are strongly influenced by obliquity fluctuations, controlling the low- to high-latitude temperature gradients, and show a remarkable similarity with a high-resolution southeast Atlantic sea surface temperature record. A comparison of the temperature profiles (site and source) and temperature gradient (DeltaT(source)-DeltaT(site)) with the non-sea-salt calcium and sodium records suggests a secondary influence of atmospheric transport changes on aerosol variations.

Holocene loess deposition in Iceland: Evidence for millennial-scale atmosphere-ocean coupling in the North Atlantic

We present the first detailed record of Holocene climate variation from Icelandic eolian soil deposits. Seven cold and windy episodes occurred in Iceland during the past 10 k.y., including the well-documented Little Ice Age (0.6‐0.1 ka) and the 8.2 ka event. These windy events are associated with enhanced drift-ice discharge into the North Atlantic, and several are associated with evidence for cold and windy climate in central Greenland and diminution of deep-water formation in the North Atlantic. Although the Arctic Oscillation‐North Atlantic Oscillation (AO‐NAO) has been invoked to explain other climate teleconnections in the North Atlantic, our paleorecord of windiness in Iceland is not consistent with a persistent negative phase of the AO‐NAO.