Pore waters of the central Pacific Ocean: Nutrient results

Interstitial waters from the MANOP siliceous and calcareous ooze sites in the central Pacific Ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler. The pore waters were analyzed for nutrients, carbonate system species, Mn 2+ , Ca 2+ , total alkaline earth metals and dissolved oxygen (siliceous ooze site only). In calcareous sediments, the ΣCO 2 , alkalinity, PO 4 3− , and Ca 2+ results obtained from shipboard techniques are significantly different than those recovered in situ. The other species measured as well as all of the data from the siliceous ooze site, reveal no significant dependence on the method used to sample the pore water. The relative changes in O 2 , NO 3 − , Mn 2+ , alkalinity, and ΣCO 2 indicate that respiration using O 2 , NO 3 − , and MnO 2 as electron acceptors is the dominant reaction affecting the pore water species. We develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments. This model includes the effects of pore water diffusion, oxygen and nitrate respiration, sediment accumulation, sediment mixing, and changes in sediment porosity. We observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles. The major factor controlling the pore water profiles is the flux of organic carbon across the sediment-water interface. We conclude that the large intra-site variability observed in the interstitial water nitrate profiles is probably caused by small-scale variations in the flux of organic material.