Nitrogen isotope profiles through the Cretaceous-Paleogene boundary; comparing records from the New Jersey coastal plain and Demerara Rise

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Author(s): Quan, T. M.; Nelson, A. J.
Author Affiliation(s): Primary:
Oklahoma State University, Boone Pickens School of Geology, Stillwater, OK, United States
Volume Title: AGU 2011 Fall Meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 Fall Meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The Cretaceous-Paleogene Boundary (K-Pg) mass extinction (∼65 Mya) resulted in the loss of a wide range of terrigenous and marine organisms. Several lines of evidence, including negative δ13C excursions in sedimentary organic and inorganic carbon profiles, indicate that the marine carbon cycle was altered across the K-Pg, but little information is known about the marine nitrogen cycle. There are also indications that post-boundary water column anoxia may have occurred, but the degree and extent of this anoxic period has not yet been determined. Since the sedimentary δ15N record can provide information on bottom water oxygen levels as well as nitrogen cycling, we have chosen to measure nitrogen isotopes across the boundary to learn more about the environmental water column conditions at the K-Pg. On geological time scales, the nitrogen cycle consists of three main processes: nitrogen fixation, nitrification, and denitrification/anammox, all of which are affected by the amount of oxygen present in the water column. The balance between nitrogen fixation and denitrification/anammox controls the pool of fixed inorganic nitrogen, and can be traced using nitrogen isotopes. If the sediment is well-preserved, the nitrogen isotopic profile can be used to assess the relative changes of denitrification/anammox and nitrogen fixation through time at a particular location. Since the sedimentary δ15N signal is also related to the deep water oxygen concentration via a simple non-linear curve, the δ15N profile can also be used to evaluate the past redox environment at a site. We have measured bulk sediment δ15N profiles from two cores: a borehole core from Bass River on the New Jersey coast (ODP 174AX) and a deep sea core from the Demerara Rise in the Western Tropical Atlantic (ODP 207). Although the two cores differ in proximity to the Chicxulub impact site as well as water depth, both δ15N records become relatively more enriched starting at the K-Pg boundary and progressing through the P3a foraminiferal zone, with a maximum enrichment within the P1c zone. This indicates a distinct relative increase in denitrification/anammox during the post-impact period, and thus would support the hypothesis that deep water oxygen concentrations at these sites decreased after the impact event. The δ15N records for both cores are more complex at the boundary itself but retain similar features between sites. Based on these cores, it appears that denitrification/anammox intensified in the North Atlantic in response to the K-Pg boundary events due to a decrease in deep water oxygen.
Year of Publication: 2011
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Atlantic Coastal Plain; Atlantic Ocean; Bass River; Biogeochemical methods; C-13/C-12; Carbon; Carbon cycle; Cenozoic; Coastal plains; Cores; Cretaceous; Demerara Rise; Denitrification; Equatorial Atlantic; Fixation; Geochemical cycle; Geochemical methods; Isotope ratios; Isotopes; Leg 174AX; Leg 207; Mass extinctions; Mesozoic; N-15/N-14; New Jersey; Nitrification; Nitrogen; Nitrogen cycle; North Atlantic; Northwest Atlantic; Ocean Drilling Program; Organic compounds; Paleo-oceanography; Paleoclimatology; Paleogene; Sediments; Stable isotopes; Tertiary; United States; West Atlantic
Coordinates: N385500 N412100 W0735300 W0753500
N080000 N080000 W0540000 W0540000
Record ID: 2015010167
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