Glacial-interglacial variations in coupled thermocline ventilation and sedimentary iron delivery to the Peru upwelling system

Primary productivity in the Humboldt Current system off Peru is limited by the supply of bioavailable iron (Fe) from reducing seafloor sediments. Previous studies have demonstrated that bottom water redox conditions exert first-order control on the Fe efflux from continental margin sediments. Fluctuations in thermocline ventilation therefore have the potential to modulate ocean fertility by altering the net efflux of Fe from the seafloor on a variety of timescales. We present a 140 ka record of high-resolution XRD core scanning, reactive Fe, redox sensitive trace metal and nitrogen isotope data for a sediment core from the present-day oxygen minimum zone off Peru. Coarser grain size as well as decreased δ¹⁵N values (≥3 ‰) and increased uranium to molybdenum ratios (≤1.6 μg g^-1/μg g^-1) indicate enhanced thermocline ventilation compared to the present-day (δ¹⁵ ≈ 6 ‰, U/Mo ≈ 0.2 μg g^-1/μg g^-1) during the LGM, MIS4, MIS5b, MIS5d and MIS6. Sediments that were deposited during these intervals are depleted in reactive Fe suggesting that the redox regime prevailing during cooler intervals fostered seafloor Fe release. The relative accumulation rate of uranium and molybdenum indicates that shifts in the Fe mobilization efficiency were related to a transition from sulfate- to Fe-reducing conditions in the sediment pore water. We suggest that lower concentrations of pore water sulfide close to the sediment surface facilitated dissolved Fe loss across the benthic boundary by decreasing Fe fixation as Fe sulfide. Our data suggest that a redox-shift toward more reducing conditions in oxygen minimum zones may not enhance but rather decrease the sedimentary Fe efflux. We will discuss implications of these findings for nitrate utilization and productivity in the ocean at glacial-interglacial transitions and in response to future de-oxygenation.