Research Highlights in the Ocean Sciences by 2005 AGU Fellows
Presiding: C R Sherwood, U.S. Geological Survey; E Druffel, University of California, Irvine
OS13A-01 INVITED 13:30h
Breakdown of Ocean Stratification during Deglaciation - Evidence from the Ocean's Si Cycle
Increased stratification of the ocean (relative to today) is one of several general mechanisms that have been proposed to explain lower atmospheric carbon dioxide concentrations during glacial times. Breakdown of this stratification during deglaciation is suggested to have released carbon dioxide trapped in the deep sea to the atmosphere. Carbon isotope records from ice cores and from marine sediment cores have been cited as evidence for this mechanism. Were this true, then one would expect to find corroborating evidence from the ocean silicon cycle. Biogenic opal (Si) is regenerated, on average, at greater depths than are carbon, nitrogen and phosphorus. Therefore, increased stratification would have trapped Si preferentially, relative to other nutrients, in the deep ocean. Breakdown of stratification would have likewise increased preferentially (relative to other nutrients) the supply of dissolved Si to surface waters in certain regions, stimulating diatom growth. In support of this idea, we find that opal burial rates at several sites around the Southern Ocean, south of the Antarctic Polar Front, increased sharply at about 16 ka BP. Within the uncertainty of the age models, we find a simultaneous increase in opal burial in the eastern equatorial Pacific Ocean (EEP). We interpret these features to be signals of the deglacial breakdown of ocean stratification. Within a single EEP core, we find that the rise in opal burial rate followed the initial drop in carbon isotopic composition of planktonic foraminifera by about 2 - 3 ka. This may reflect a phased breakdown of ocean stratification in which intermediate waters were ventilated much earlier than deep waters. Whether or not a phased breakdown is born out by further work, the initial rise in opal accumulation, both in the Southern Ocean and in the EEP, seems to have occurred when North Atlantic Deep Water formation was at a minimum (i.e., during Heinrich Event 1), indicating a Southern Ocean source for the reinvigorated ventilation of the deep sea.
OS13A-02 INVITED 13:55h
Paleo-ocean chemistry based on records in marine sedimentary opal: implications for effect of Fe and other trace elements on biological productivity
We report on our discovery that marine opal contains a high fidelity record of dissolved oceanic concentrations of cosmic ray-produced radionuclides, 10Be and 26Al, while also capturing temporal variations in a large number of trace elements such as Ti, Fe, Zn, and Mn. This finding is based on our studies of trace elements (and two cosmogenic nuclides) in biogenic opal, over the last full ice age cycle, in opal derived from the site 1093 and its companion piston core TN057-13 (49o 59'S, 5o 52'E), near the present-day position of the Antarctic Polar Front. The data show potential for determining the controls on global ocean productivity -- in particular, the extent to which marine production can be modulated by external sources of micronutrients. Despite clear evidence for iron limitation in the modern ocean, the sedimentary record of "paleoproductivity" has not as yet offered any clear picture of the possible relationship between the changes in dust flux (known to have occurred over ice age cycles, for example), and regional or global productivity. Thus, with one sedimentary phase and in single sedimentary sections, we now have the potential to compare directly a proxy for aeolian input of micronutrients (e.g. Fe or Ti), with a proxy for production (e.g. 26Al/Al ratios). We expect that studies of the temporal records of trace elements and cosmogenic nuclides in contrasting regions of upwelling and productivity, which exhibit different sensitivities to global climate fluctuations and micronutrient inputs, would lead to a direct and comprehensive test of ideas such as Martin's hypothesis of iron control of atmospheric carbon dioxide Martin (1990), in different oceanic provinces during glacial cycles. Reference: Martin, J.H. Glacial-interglacial CO2 change: the iron hypothesis. Paleoceanography 5, 1-13, 1990.
OS13A-03 INVITED 14:20h
The Mid-Latitude Westerlies, Atmospheric CO2, and Climate Change during the Ice Ages
An idealized three-dimensional model is constructed of the ocean's deep circulation and CO2 system that reproduces the main features of glacial-interglacial CO2 cycles, including the correlation between atmospheric CO2 and Antarctic temperatures, the lead of Antarctic temperatures over CO2 at terminations, and the shift of the ocean's d13C minimum from the North Pacific to the Atlantic sector of the Southern Ocean. The key feature of the model is in the atmosphere instead of the ocean. The key feature is a relationship between the position of the mid-latitude westerly winds, atmospheric CO2, and the mean state of the atmosphere. Cold glacial climates seem to have equatorward-shifted westerlies, which allowed more organically cycled CO2 to accumulate in the deep ocean. Warm climates like the present have poleward-shifted westerlies that flush the organically cycled CO2 out of the deep ocean.
OS13A-04 INVITED 14:45h
Dynamics of Thermohaline Circulation Variability
The processes of deep water formation in the subpolar North Atlantic show a considerable amount of variability which leads to variations of the Atlantic Thermohaline Circulation on interannual and decadal time scales. The relevant physical mechanisms will be discussed, with a particular emphasis on the ability of ocean circulation models to simulate aspects of the observed variability.