Nitrogen isotope variations in Santa Barbara Basin sediments: Implications for denitrification in the eastern tropical North Pacific during the last 50,000 years

Nitrogen isotope variations preserved in Santa Barbara Basin sediments are used to evaluate changes in denitrification in the eastern tropical North Pacific (ETNP) during the last 50,000 years. A significant component of the subsurface waters (similar to 100-400 m) that presently fill the Santa Barbara Basin is derived from the low-oxygen, denitrifying zone in the ETNP, and the nitrate in these waters has a delta(15)N value of 8-9 parts per thousand. During the last glacial, the delta(15)N values of Santa Barbara Basin sediments were typically 6-7 parts per thousand, indicating decreased denitrification in the ETNP and a better oxygenated intermediate water mass in the Santa Barbara Basin at this time. This reduced denitrification during the last glacial would have increased the poor of fixed nitrogen and may have contributed to the higher productivity previously reported for various regions of the global ocean during this period. At the onset of deglaciation, sediment delta(15)N values increase by more than 2 parts per thousand, indicating increased denitrification in the ETNP. During Younger Dryas time, delta(15)N values decreased by 3 parts per thousand and record a brief return to better ventilated conditions in the subsurface waters of the ETNP. This is followed by an increase in delta(15)N to over 9 parts per thousand at similar to 10,000 years ago, indicating intense denitrification at the beginning of the Holocene.