Fertilizing the Ocean Deserts During the LGM: Is There Evidence for Increased Paleoproductivity and Redfield Decoupling in the Glacial Tropical/Subtropical World Ocean?

Twenty-five cores were taken from all three tropical and subtropical ocean basins to evaluate changes in surface water productivity as a function of Dd13Ccalcite G. ruber (LGM-HOL) of the low latitude biological pump in a cool glacial world. Corrections applied to the data included a -0.32% carbon reservoir effect, as well as correcting for temperature dependent atmospheric-oceanic CO2 equilibrium. Evidence for productivity in excess of Redfield Dd13Ccalcite G. ruber : [PO4- ] limitations is taken where Dd13Ccalcite G. ruber (LGM-HOL) in excess of Redfield = Dd13Ccalcite G. ruber (LGM-HOL) -0.93 _%umol*kg [PO4- ]. Dd13Ccalcite G. ruber distributions allow for three main conclusions regarding the strength of the biological pump in tropical and subtropical HNLP and LNLP zones during the Last Glacial Maximum: 1.) Nutrient fronts associated with HNLP regions today expanded further into subtropical oligotrophic gyres, where iron deposition in a dustier glacial world freed phytoplankton from iron limitation, thus allowing for enhanced carbon export out of surface waters. Almost all sites demonstrated increased export in these regions in accordance to and within detectable bounds of presently understood Redfield nutrient dynamics. 2.) Evidence for decoupling of Redfield occurs in the oligotrophic world ocean along HNLP-LNLP transitional niche ecotomes. Redfield decoupling in these zones can be associated with sites of large diatom paleo-depositional events limited in spatial extent, but very intense in production through time during the Last Glacial. Diatom species associated with large Dd13Ccalcite G. ruber isotopic excursions beyond Redfield constraints are non-obligate diazotrophs Rhizosolenia sp and Ethmodiscus Rex. Decoupling of Redfield at these sites leaves unique isotopic signatures in marine sedimentary records despite lack of preservation for other diazotrophs such as cyanobacterial Trichodesmium sp. 3.) Despite the presence of smaller diazotrophs in the modern LNLP ocean, productivity within deeply oligotrophic gyres operate at steady state export on glacial-interglacial timescales as a consequence of a deeper mixed layer, thus making phosphate reserves inaccessible to larger diazotrophs. Correlations between phosphocline depth and changes in productivity over glacial-interglacial timescales suggest that productivity changes track mixed layer depth. Mixed layer depth should be a function of atmospheric-surface ocean feedback, secondary to sustained wind strength. If intensity of export productivity tracks both thermocline and phosphocline depth, then rates of carbon fixation by increased efficiency of phosphate uptake due to lifting of iron limitation as well as rates of non-Redfield productivity increases should oscillate and lag changes in the hydrological cycle on both Milankovitch and suborbital timescales. Spatial correspondence between occurrences of diatom mats of Rhizosolenia sp. and Ethmodiscus Rex to export productivity in excess of Redfield constraints on carbon fractionation is encouraging for arguing enhancement of both the biological pump, and possibly, N-fixation in a cooler glacial world.Increases in the silica pump allow for greater pCO2 drawdown without corresponding changes in ocean alkalinity. Whether or not such increases in export productivity at low-latitudes can actually account for PCO2 drawdown remains to be modeled.