Geochemistry Of Diatom Mats From Tropical West Pacific: Implications For Carbon And Silicon Cycle And Response To Paleoceanographic Conditions

Diatom mats play an important role in changing partial pressure of atmospheric carbon dioxide (PCO2) and, consequently modulating global climate by massive flux of organic carbon and biogenic silicon to seafloor and improved efficiency of marine biological pump. However, their significances in global carbon and silicon cycles have not heretofore been sufficiently emphasized. Two sediment cores (WPD-03 and WPD-12) with laminated Ethmodiscus rex diatom mats (LDM), located in the Parece Vela Basin of the eastern Philippine Sea, were used to trace the source and relative utilization degree of nutrient Si for the blooming of Ethmodiscus rex, evaluate paleoproductivity levels and reconstruct paleoredox environments during the LDM deposition by measurements of clay minerals, biogenic components, major, trace and rare earth elements, total organic carbon isotope, and Ethmodiscus rex carbon and silicon isotopes. Furthermore, we elucidated the deriving force of LDM in tropical carbon cycle and, finally explored the paleoceanographic constraints on the formation and deposition of LDM from tropical West Pacific.Clay mineral and Ethmodiscus rex silicon isotope compositions show that, during the diatomaceous clay (DC) deposition, it recorded a remarkable strengthened eolian accumulation, which imported plentiful silicon and iron to promote the blooming of Ethmodiscus rex probably and consequently resulted in the formation of LDM. Special ecological characteristics (i.e. populating in subsurface seawater) and peculiar demands on marine environments (i.e. adapted to stratified seawater) of Ethmodiscus rex could result in the lag response of their blooming to dust inputs.With the isotope fractionation model of diatom for dissolved silicon utilization, Ethmodiscus rex silicon isotope composition suggests that Ethmodiscus rex completely consumed the silicic acid during the Last Glacial Maximum (LGM) of LDM deposition. As a result, for the nutrient silicic acid, the eastern Philippine Sea was characterized by oligotrophic conditions during the LGM, the same as at present.Biogenic component, total organic carbon isotope and Ethmodiscus rex carbon isotope compositions indicate that paleoproductivity increased through time during deposition of the pelagic clay (PC), DC, and basal LDM, followed by a stabilization during deposition of middle and upper LDM. Primary productivity, organic carbon rain rates, burial productivity during the LDM deposition were averagely estimated to 248.42 g/m2 yr, 61.93 g/m2 yr and 5.27 g/m2 yr, respectively. Thereinto, the primary productivity is comparable to those of some modern continent-margin upwelling zones. Moreover, the estimations for several kinds of productivity above suggest that a caution must be taken to assess the'deep'productivity represented by giant and'shade flora'diatoms, which should be distinguishable from classic'surface'productivity characterized by common phytoplankton in the photic zone.Major, trace and rare earth element compositions infer that the LDM, DC and PC accumulated under sulfidic anoxic, suboxic and oxic conditions, respectively, while redox conditions in eastern Philippine Sea bottom waters during the LDM deposition were probably largely suboxic. Sulfidic anoxic conditions within the LDM appear to have coincided with marine productivity maxima, thus suggesting that oxygen depletion in the pore water and at the seawater-sediment interface may have been linked to an increased flux of organic matter to the seafloor rather than to restricted lateral circulation. Ethmodiscus rex carbon isotope compositions suggest that the blooming of Ethmodiscus rex during the LGM consumed plenty of dissolved CO2 in seawater (CO2(aq)). In the DC-LDM transition, the CO2(aq) content rapidly decreased from 15.4μmol/L to 6.5μmol/L; Accordingly, PCO2 also distinctly reduced from 545 ppmv to 231 ppmv. Average PCO2 of about 220 ppmv during the LDM deposition is markedly lower than that of about 440 ppmv during the DC deposition, and PCO2 gradually decreased to the level evident lower than LGM global PCO2 arranging from 180 to 200 ppmv during deposition of upper LDM, suggesting LDM played a substantial'carbon sink'role, and finally developed the eastern Philippine Sea into an obvious sink for CO2.Integrated analysis suggest that mat-forming, giant and'shade flora'diatom Ethmodiscus rex utilized dust silicon, bloomed in the stratified seawater with the stimulation of dust Fe and then deposited to LDM by the'fall dump'in the eastern Philippine Sea. Available data suggest that the LDM appear not to have been associated with frontal zone, and no dissolved silicon from the Southern Ocean described as the silicic acid leakage hypothesis promoted the blooming of Ethmodiscus rex in the eastern Philippine Sea.