Nitrogen Isotope Ratios in Sedimentary Organic Matter Track Changes in Nutrient Utilization and Inventories

S. E. Calvert and T. F. Pedersen

The isotopic composition of sedimentary nitrogen is a direct proxy for the extent of nutrient draw-down in surface waters and for changes in nutrient inventories in past oceans. In the eastern Pacific, surface sediment ^dgr¹⁵N is lowest along the equator where surface ocean nitrate concentration ([NO₃-]) is highest; ^dgr¹⁵N progressively increases and [NO₃-] decreases to the north and south. This pattern is produced by the formation and sedimentation of ¹⁴N-enriched organic material (OM) where phytoplankton growth is not limited by NO₃-, and the formation of isotopically heavy OM where NO₃- is progressively drawn down. Downcore records from the Panama Basin and Northwest Africa show a 1 to 2^pmil decrease in ^dgr¹⁵N from the Holocene (0 to 12 ky BP) to the last glacial maximum (LGM, 12 to 24 ky BP) due to a decrease in relative NO₃- utilization (biological uptake relative to supply) and an increase in OM burial rate due to an increase in productivity. Off northwestern Mexico and in the Arabian Sea, extensive denitrification in intermediate waters causes ¹⁵N-enrichment of the source NO₃-, so that modern sediment ^dgr¹⁵N is high. During the LGM, however, ^dgr¹⁵N decreased because productivity and the extent of denitrification both fell. Hence, the global ocean NO₃- inventory increased, possibly leading to higher global productivity which could have been responsible for the lower glacial atmospheric pCO ₂.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California