Glacial‐Holocene δ15N record from the Gulf of Tehuantepec, Mexico: Implications for denitrification in the eastern equatorial Pacific and changes in atmospheric N2O

Abstract

The intensity of denitrification in the Gulf of Tehuantepec, along the Mexican margin of the eastern tropical North Pacific (ETNP), was reconstructed for the last 23,000 years using the nitrogen isotopic composition of sediment organic matter (δ15Norg) in conjunction with other geochemical tracers of water column productivity. During the last glacial period, δ15Norg values ranged from 6.5 to 8.6‰ with values increasing between 23 and 17 ka. Deglaciation was marked by an abrupt increase in δ15Norg with maximum values (9.9‰) occurring during the Bølling/Allerød period and the earliest Holocene. A decline in δ15Norg values occurred over the last 10,000 years, with late Holocene values being similar to those of the glacial period. Our data suggest that denitrification in this region was minimal during the last glacial and is consistent with the deposition of bioturbated sediments at the study site. The intensity of denitrification increased dramatically during deglaciation and the early Holocene, signaling the presence of a strong oxygen minimum zone. This period is also marked by the deposition of laminated sediments in the region. The δ15Norg records display similar temporal trends to those seen in the Greenland ice core record of atmospheric N2O concentrations. At present, denitrification within the eastern tropical Pacific and Arabian Sea accounts for a major portion of the global marine loss of fixed nitrogen, and a significant fraction of the ocean‐atmosphere N2O flux originates from these areas. Large changes in N2O emissions from these regions in the past would have directly impacted global greenhouse gas concentrations.