Reconstruction of nitrate utilization rate change based on diatom-bound nitrogen isotope values in the central slope area of the Bering sea during the early pleistocene (2.4−1.25 Ma)

Abstract

Because the high latitude region in the North Pacific is characterized by high primary production in the surface water enriched with nutrients, it is important to understand the variation of surface water productivity and associated nutrient variability in terms of global carbon cycle. Surface water productivity change or its related nutrient utilization rate during the Northern Hemisphere Glaciation (NHG; ca. 2.73 Ma) has been reported, but little is known about such circumstances under gradual climate cooling since the NHG. Bulk nitrogen isotope (δ15Nbulk) of sedimentary organic matter has been used for the reconstruction of nutrient utilization rate in the surface water. However, sedimentary organic matter experiences diagenesis incessantly during sinking through the water column and after burial within the sediments. Thus, in this study we examine the degree of nitrate utilization rate during the early Pleistocene (2.4−1.25 Ma) since the NHG, using the diatom-bound nitrogen isotope (δ15Ndb), which is known to be little influenced by diagenesis, from Site U1343 in the Bering slope area. δ15Ndb values range from ~0.5 to 5.5‰, which is lower than δ15Nbulk values, but they vary with larger amplitude. Variation patterns between δ15Ndb values and biogenic opal concentration are generally consistent, which indicates that the nitrate utilization rate is closely related to opal productivity change in the surface water. A positive correlation between opal productivity and nitrate utilization rate was observed, which is different from the other high latitude regions in the North Pacific. The main reason for this contrasting relationship is that the primary production in the surface water at Site U1343 is influenced mostly by the degree of sea ice formation. Still, although concerns about diagenetic alteration have been avoided by using δ15Ndb, the effects of the preservation state of biogenic opal and the speciesdependent isotopic fractionation on δ15Ndb should be assessed in the future studies.