Using 15N/14N in Diatom Fossils to Understand Changes in Surface Ocean Nutrient Dynamics 2.73 MA

Abstract

The climate transition from the warm Pliocene to the Pleistocene epoch of ice age cycles was associated with a marked step at approximately 2.73 Ma, when Northern Hemisphere glaciers reached sea level and discharged icebergs with rock debris into the ocean, a change clearly recorded in open ocean sediment records. In the Western subarctic Pacific, this change was also marked by a decrease in opal mass and alkenone accumulation and an increase in the 15N=14N ratio of organic matter bound within diatom fossils (d15Ndb), the latter indicative of a greater degree of nitrate consumption in the surface ocean. The coupling of an apparent decline in export production and a rise in the degree of nitrate consumption calls for a decrease in nitrate supply from below, possibly due to reduced wintertime deep-mixing in the Western subarctic Pacific, the region of the subarctic Pacific in which wintertime deep mixing and nitrate recharge is most intense today. However, preliminary data from a core in the Southeast subarctic Pacific, SO202-33-4, indicate a decrease in d15Ndb at or near the 2.73 Ma transition. In this study, we improved the resolution of the record, while measurement discrepancies led to an investigation of the isotopic differences among the different size fractions of diatom opal separated from the sediments. With a preliminary age model, we find that d15Ndb drops from 12.7h to 11.4h at 2.73 Ma. We hypothesize that this decrease in d15Ndb was caused by the same mechanism of reduced deep-mixing in the Western subarctic that caused the d15Ndb to increase in the Northwest. This hypothesis is based on isotopic evidence from the modern ocean that nitrate is imported from below in the Northwest, undergoes consumption and d15N increase, and is then transported to SO202-33-4 via surface currents, raising the d15N of the nitrate supply to the southeast subarctic Pacific. Reduced deep-mixing in the Northwest at 2.73 Ma would have led to a decline in the amount of elevated d15Nnitrate that is transported to site SO202-33-4. Over the 2.73 Ma transition, the dominant source of nitrate at SO202-33-4 would have shifted from lateral transport to local wintertime vertical mixing, with the local vertical mixing supplying subsurface nitrate with a lower d15Nnitrate (6.0h). Size fraction d15Ndb and smear slide analysis of the diatom opal revealed that the current separation protocol tends to remove both very large and very small diatoms. Since the diatom species composition changes across the 2.73 Ma transition, this affects the samples to varying degrees, with more small Neodenticula diatoms removed from samples younger than 2.73 Ma. As different size fractions have different d15Ndb, the species bias affects the total d15Ndb results.