Evaluating foraminifera-bound δ 15 N as an ocean deoxygenation proxy: The influence of oxygen-deficient zone depths

Tianshu Kong¹Thomas Lee¹Kameko Landry¹Ruixiang Zhai¹Sijia Dong²Xingchen Wang^1*

• ¹Boston College, Chestnut Hill, United States
• ²Nanjing University, Nanjing, Jiangsu Province, China

The nitrogen isotopic composition of shell-bound organic matter in planktonic foraminifera (FB-δ15N) is widely used as a proxy for past ocean deoxygenation because water-column denitrification in oxygen-deficient zones (ODZs) preferentially removes 14N, enriching the remaining nitrate in 15N. Typically, increases in FB-δ15N records from ODZ-influenced regions are interpreted as evidence of ODZ expansion or intensification. However, planktonic foraminifera predominantly feed on organic nitrogen derived from the subsurface nitrate immediately below the euphotic zone, often above ODZ core depths. It remains unclear if the δ15N maxima observed within ODZ cores, reflecting denitrification intensity at a given location, directly correlates with the FB-δ15N values recorded above. Here, we combine new and published data from the eastern tropical Pacific ODZs to examine relationships among subsurface nitrate δ15N, ODZ δ15N maxima, and ODZ upper-boundary depths. Our analysis reveals a strong correlation between subsurface nitrate δ15N and ODZ δ15N maxima (R2 = 0.56-0.79), supporting the use of FB-δ15N as an indicator of denitrification intensity within ODZ regions. However, subsurface nitrate δ15N also correlates strongly with the ODZ upper-boundary depth (R2 = 0.57-0.59), with lower δ15N values observed where ODZs are deeper. For example, at our new study sites in the Eastern Tropical North Pacific (5 – 8°N), where the ODZ upper-boundary depth is ~300 m, the δ15N maxima (>10‰) at the ODZ core decrease upward to subsurface nitrate δ15N values of ~6.5‰ — only slightly higher than the global pycnocline nitrate δ15N. These results suggest that variations in ODZ depth should be accounted for when interpreting FB-δ15N records (and other δ15N archives) from ODZ regions. Under warmer conditions, organic matter remineralization may become shallower due to the temperature dependence of respiration, shifting ODZs upward and elevating FB-δ15N even without changes in denitrification rates. To more robustly reconstruct ODZ history using FB-δ15N, we recommend using multiple sites from the ODZ interior to regions beyond their modern boundaries. Cores situated outside modern ODZs, where thermocline nitrate δ¹⁵N still carries the ODZ signature, are ideal for tracing ODZ expansions and contractions, while cores from within the modern ODZs provide complementary constraints on ODZ intensity and vertical structure.