Foraminifera-bound nitrogen isotopes over the past 25 million years

Abstract:

The million-year variability of the marine nitrogen (N) cycle during the Cenozoic era is poorly understood. A recent study has reported the first measurements of planktic foraminifera-bound N isotopes (FB-δ¹⁵N) from 70 to 30 million years ago (Ma) (Kast et al., 2019). Here, we report the first FB-δ¹⁵N measurements spanning the last 25 million years from three sediment cores located in the subtropical gyres of the South Atlantic (DSDP 516), Pacific (ODP 872) and Indian Ocean (ODP 754). Combined with our previous work, a first view is arising of a whole-Cenozoic FB-δ¹⁵N record, recognizing that regional variation in δ¹⁵N is important and calls for the generation of multiple records. Our data indicate that major shifts in the marine N cycle occurred during the Miocene period. During the Middle Miocene Climatic Optimum (~17-15 Ma), FB-δ¹⁵N values dropped in the Pacific by ~10 ‰ and Atlantic by ~5 ‰, causing a significant reduction in the δ¹⁵N gradient between the two ocean basins. We propose that these changes are best explained by a combination of two changes. The first was an increase in sedimentary denitrification rate, caused by the expansion of continental shelves associated with the MMCO sea level rise and by an increase in nutrient input to the shelves due to enhanced continental weathering. An increase in N fixation would have occurred in response to the increase in sedimentary denitrification, with the net result being a lower global mean ocean nitrate δ¹⁵N. The second proposed change was a decrease in water column denitrification caused by tectonically-driven ocean circulation changes that increased the supply of oxygen to the subsurface ocean, which may have reduced the Pacific-to-Atlantic δ¹⁵N difference while providing a second mechanism for lowering mean ocean nitrate δ¹⁵N. After the MMCO, Atlantic FB-δ¹⁵N remained relatively stable while the Pacific FB-δ¹⁵N progressively increased, resulting in the redevelopment of a strong N-isotope gradient between the two basins. We hypothesize that these changes were driven by a return to rapid water column denitrification in the Pacific associated with the gradual closing of low-latitude oceanic gateways during the Miocene. The data imply that an ocean state with substantially higher water column denitrification rates than modern was a common occurrence over the Cenozoic, waxing and waning with bathymetric changes.

Reference:
Kast, E. R., Stolper, D. A., Auderset, A., Higgins, J. A., Ren, H., Wang, X. T., ... & Sigman, D. M. (2019). Nitrogen isotope evidence for expanded ocean suboxia in the early Cenozoic. Science, 364(6438), 386-389.