The δ ¹⁵ N of chlorophyll to reconstruct the nitrate cycle in Adélie Land, East Antarctica, over the last 2000 years

Abstract. The highly productive Antarctic coastal waters are a key component of the strong Southern Ocean biological pump, supported by high nutrient availability. However, modern observations of the nitrogen cycle and phytoplankton responses in these regions remain limited, particularly at multi-decadal to centennial timescales. The use of nitrogen stable isotopes (δ15N) measured in chlorophyll a (Chl a) preserved in marine sediment offers a new opportunity to better understand the relationships between the past primary productivity, nitrate supply and environmental conditions. Because the δ15Nchl is directly derived from phytoplankton and is not affected by diagenetic alteration, it provides valuable insights into long-term changes in the nutrient cycle. Here we present the first antarctic δ15Nchl record from the well-dated U1357B IODP Site located offshore Adélie Land, East Antarctica, spanning the last two millennia. Our δ15Nchl record shows a strong variability with isotopic values oscillating between –6 ‰ and –2 ‰. Comparison with other proxy reconstructions reveals periods of higher δ15Nchl at ~1850–1500 yrs BP and ~1100–500 yrs BP, corresponding to enhanced sea-ice cover and late seasonal melting. In contrast, lower δ15Nchl at 1500–1100 yrs BP and since 500 yrs BP coincide with less sea ice extent and earlier retreat. We interpret these variations in δ15Nchl as reflecting changes in nitrate supply from the subsurface nitrate-rich modified Circumpolar Deep Waters, driven by variations in sea-ice and atmospheric conditions over the last 2000 years.