Support From the Southern Ocean and Equatorial Pacific for the Glacial Shelf-Nutrient Hypothesis

The impact of sea level variations on marginal sedimentation and hence biogeochemical sinks has long been recognized, but few quantitative data exists on the oceanic response of the biolimiting nutrient phosphorus (P) to changing depositional sinks. A greater understanding of the sinks and geochemistry of P in marine sediments, together with multi-proxy analyses of a variety of P-related records from the deep sea, indicates that the redistribution of this nutrient from marginal to deep sea sinks during glacial lowstands has had an important impact on the oceanic P cycle. We compared records of P/Ti ratios (an "excess" P proxy) and P accumulation rates from the Southern Ocean (ODP Site 1089) and the eastern equatorial Pacific. The P/Ti records for these widely spaced sites show a remarkable degree of similarity over the last 400 kyr. The P/Ti ratios at both sites exhibit peaks with a 100 kyr periodicity, and a phasing of broad peaks that begins during glacial intervals, reaching maxima just after the glacial-interglacial transition, then decreasing to low values by the beginning of the next glacial interval. These records indicate relatively high "excess" P export occurring about 40-60 kyr after the onset of glacial intervals. P accumulation rates from these sites reveal sharp peaks 40-60 kyr after glacial onset, followed by a slow decline, with some abbreviated peaks, over the ensuing 60-80 kyr. These two semi-independent proxies provide a picture of a potentially globally-coherent signal in the deep ocean marine P mass balance, with increased P export to the deep ocean from shelf weathering and lack of shelf depositional area during glacials. The delay in P accumulation in the deep ocean is due to the slower response time of P (residence time of 10-20 kyr). Given higher recycling of P in the deep sea sink, this redistribution from shelf to deep sea may have resulted in higher oceanic dissolved phosphate concentrations and increased oceanic productivity, especially soon after glacial onset.