Abstract
Paleoproductivity is a critical component in past ocean biogeochemistry, but accurate reconstructions of productivity are often hindered by limited integration of proxies. Here, we integrate geochemical (phosphorus) and micropaleontological proxies at millennial timescales, revealing that the coccolithophore record in the Subantarctic zone of the South Atlantic Ocean is driven largely by variations in marine phosphorus availability. A quantitative micropaleontological and geochemical analysis carried out in sediments retrieved from Ocean Drilling Program Site 1089 (Subantarctic Zone) reveals that most of the export productivity in this region over the last 0.5 my was due to coccolithophores. Glacial periods were generally intervals of high productivity, with productivity reaching a peak at terminations. Particularly high productivity was observed at Termination V and Termination IV, events that are characterized by high abundance of coccolithophores and maxima in the phosphorus/titanium and strontium/titanium records. We link the increase in productivity both to regional oceanographic phenomena, i.e., the northward displacement of the upwelling cell of the Antarctic divergence when the ice-sheet expanded, and to the increase in the inventory of phosphorus in the ocean due to enhanced transfer of this nutrient from continental margins during glacial lowstands in sea level. The Mid-Brunhes interval stands out from the rest of the record, being dominated by the small and highly calcified species Gephyrocapsa caribbeanica that provides most of the carbonate in these sediments. This likely represents higher availability of phosphorus in the surface ocean, especially in mesotrophic and oligotrophic zones. Under these condition, some coccolithophore species developed an r-strategy (opportunistic species; growth rate maximized) resulting in the bloom of G. caribbeanica. These seasonal blooms of may have induced “white tides” similar to those observed today in Emiliania huxleyi.
Highlights
AND OBJECTIVES Reconstructing the players and the drivers of past oceanic productivity is critical for understanding the role that nutrients and nutrient cycling, ecological structure, and temperature play in past, and perhaps future, oceans
Several authors have correlated the calcium carbonate record to paleoproductivity (Rühlemann et al, 1999; Moreno et al, 2002). From this perspective and given the longer-term nature of the record we study here, peaks in coccolithophore accumulation of coccolithophores must be related to increases in general primary productivity
The abundance of coccolithophores in the present day and fossil records is higher in eutrophic areas than that in oligotrophic areas (Kleijne et al, 1989; Flores et al, 1995; Giraudeau and Bailey, 1995); coccolith concentrations in the sediments may be the result of high coccolithophore productivity in surface waters, low coccolith dissolution in water, or at the sediment/water interface, or both
Summary
AND OBJECTIVES Reconstructing the players and the drivers of past oceanic productivity is critical for understanding the role that nutrients and nutrient cycling, ecological structure, and temperature play in past, and perhaps future, oceans. Some key components are not well-reflected or well-understand (e.g., Filippelli et al, 2007), including the role of the past global phosphorus cycle on biological productivity and the history and distribution of coccolithophores, a ubiquitous group in modern and past oceans but one whose relevance to paleoceanographic reconstructions has been under-appreciated (e.g., Flores et al, 2003) This contribution hopes to address both of these issues by comparing proxies for the marine phosphorus cycle with coccolithophore assemblage data through time in the critical Southern Ocean (SO), a region critical on glacial/interglacial timescales as a key biological pump of carbon. In high latitudes, the absence of species like F. profunda preclude the use of some of these techniques, www.frontiersin.org
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