The Evolution Of Biological Productivity And Sedimentary Fe Speciation In Eastern Equatorial Pacific Since Late Miocene

Eastern Equatorial Pacific(EEP)is an important upwelling region of the global oceans.Wind-driven upwelling of subsurface seawaters brings a large amounts of nutrients and inorganic carbon to the surface and lead to a net release of CO₂ into atmosphere,EEP thus becomes a significant source of atmospheric CO_2.However,EEP becomes a typical Fe-limited sea area because the bioavailable Fe in its surface waters is far from meeting the needs of biological primary production.Diffetent degrees of Fe limitation will affect the structure of biological community in surface waters.This will result in changes in the composition and flux of biological particles exported to deep water and further affect global carbon cycle.Marine environment and biological productivity of EEP have changed significantly over the past few million years since late Miocene,but the relationship between biological production and Fe limitiation has been poorly studied.Here we selected marine sediments of ODP 1241 and ODP 677 from Eastern Equatorial Pacific,with ages ranging from?8 Ma and?6 Ma,respectively,to reconstruct the changes of biological productivity since late Miocene by burial fluxes of multiple productivity indicators(opal,organic carbon,excess Ba,calcium carbonate).At the same time,we explored the relationship between iron input and the biological production of the two cores by using sequential chemical leaching of Fe speciation,the analysis of pyrite by chromium reduction and the analysis of major and trace elements.Due to the strong sulfidation of pore wter of ODP 677,productivity indicators such as Ba_exhad undergone significant dissolution and loss,but it provides a new constraint for the characteristics of sedimentary iron speciation under the influence of high productivity and hydrothermal activities.The Fe/Al ratio of ODP 677 is about 0.78±0.14(n=35,1 SD),significantly higher than the average ratio of 0.44 in the upper continental crust,whi le Al/Ti ratio is close to the upper continental crust and remains stable at 22.4±1.3(n=35,1 SD).It is inferred that a considerable amount of Fe is derived from adsorption onto lithogenic particles or from the self-aggregation/precipitation of dissolved Fe in the deep water column under the background of active hydrothermal activity and high productivity.The ratio of reactive Fe to total Fe(Fe _HR/Fe _T)is about 0.30±0.10(n=35,1 SD),which is much lower than the expected input flux of reactive Fe,indicating that some reactive Fe may be converted to less reactive components under high productivity.At the same time,a part of the reactive Fe is pyritized,and the ratio of pyrite Fe to total reactive Fe(Fe_py/Fe _HR)is about 0.39±0.14(n=35,1 SD),with the highest ratio of about 0.7.A strong biological productivity increase is recorded in ODP 677during 1.5?2.2 Ma,with organic carbon content reaching up to 4.61%.The high Mo and U fluxes indicate that pore water in the early diagenesis at this period may be in a highly sulfidic environment,but the high burial flux of pyrite or the place with high degree of pyritization occur at deeper depths of the core,indicating that some of the reduced S and Fe in pore water may migrate and precipitate at deeper depths.Compared with the reported Fe speciation of global marine sediments in the oxic bottom water environment,some of our samples in ODP677 exhibit high degree of concurrent increase in Fe_py/Fe _HR and Fe _HR/Fe _T,which is consistent with the typical“ferruginous ocean”depositional characteristics.We thus emphasize the importance of understanding the depositional environment and distinguishing the potential superimposed effects of high productivity and hydrothermal activities when applying Fe speciation,in order to reliably interpret the redox state of the deep ocean in the geological past.By contrast,the variation of each productivity indicators of ODP 1241 is consistent and the pyritization was weak,so the influence of Fe on biological production and biological carbon pump can be reliably tested on tectonic time scale.The biological production history of ODP 1241 over the past 8 Ma can be divided into two main stages:(1)Late Miocene biogenic bloom(LMBB)stage from 8 Ma to 4.5 Ma,during which the fluxes of various indicators were relatively high,but not completely coordinated;(2)The stage of relatively low biological production from 4.5 Ma to the present.There is no correlation between total Fe _T flux,non-lithogenic Fe flux or highly reactive Fe flux and each productivity indicator of ODP 1241,indicating that the input of Fe didn't drive the change of phytoplankton primary productivity,and the higher export production,the stronger the degree of Fe limitation.During LMBB period,ODP 1241 recorded a synchronous increase in Opal/Ba_ex ratio,revealing an increase in the silicification efficiency of diatiom frustules(or a decrease in organic carbon/diatom output),consistent with previous understanding of primary production and Fe limitation in modern upwelling zones.In late Miocene,the enhanced upwelling and/or reorganization of oceanic nutrients maintained the high biological productivity,which may lead to a further incease of Fe limitation,stimulated the increase of diatom silicification in upwelling area,and inhibited the consumption of inorganic caron and other nutrients.The decrease of biological carbon pump efficiency during LMBB weakened the ability of seawater to absorb atmospheric CO₂.