Efficiency of Biological Carbon Pump: Insights from Compound-Specific Amino Acid δ15N

As a vital component of the global carbon cycle, the ocean’s biological carbon pump determines the amount of carbon fixed by primary production in the surface waters. The efficiency of this biological pump is closely interconnected with the nitrogen cycle, which regulates nutrient inventory and primary productivity rates. Additionally, the structure of the upper ocean ecosystem influences the efficiency of the biological pump by determining how much fixed carbon is exported to the deep ocean. Compound-specific nitrogen isotope (δ¹⁵N_AA) is a novel proxy that could provide valuable insights into both aspects of these questions. The δ¹⁵N_AA could unravel isotopic information of source nitrogen and show δ¹⁵N changes associated with trophic processes. This study generates sedimentary δ¹⁵N_AA, as well as bulk sediment δ¹⁵N and organic δ¹⁵N records from a western equatorial Pacific site (MD10-3340) covering the last 140 kyr. Our results demonstrate an overall agreement among three proxies, all indicating distinct precessional variations in source nitrate δ¹⁵N and potential changes in nutrient inventory. More importantly, the δ¹⁵N_AA signatures suggest an inverse relationship between animal trophic activity in the surface water and the degradation of organic matter precipitating through the water column. Higher ecosystem trophic position is found during glacial periods, accompanied by inactive organic matter recycling, which implies a greater potential for carbon burial in deep reservoirs. Together, we suggest that the δ¹⁵N_AAsignatures can provide a detailed picture of carbon cycle coupled with nitrogen cycle.