| 10 Apr 2026
The δ15N 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.
Major comment:
Sutre et al present a new Southern Ocean d15N record based on chlorophyll, which, to the best of my knowledge, is the first of its kind and the first d15N record in the Southern Ocean to cover the last 2000 kyr. As such, the dataset is sufficiently novel and deserves publication in a journal such as Biogeoscience. However, several clarifications are required before the manuscript can be accepted.
First, the discussion of the factors controlling chlorophyll d15N, in Section 5.1, should be rewritten in light of the existing literature. It is unclear why the authors focus on N2 fixation and denitrification, given that these processes are not considered to control nitrate d15N (and exported d15N) in the Antarctic Zone. Several studies, including (Altabet and Francois, 1994; DiFiore et al., 2009; Fripiat et al., 2019; Sigman et al., 1999), have shown that nitrate d15N, and exported organic d15N, are controlled to first order by the degree of nitrate consumption and the associated isotopic discrimination.
I would also recommend a more detailed discussion of the offset between chlorophyll d15N and exported organic d15N. First, the observed offset appears broadly consistent with literature estimates, for example 5.3 ± 1.6‰ in Sachs et al. (1999), and this should be clearly discussed. Second, Sachs et al. (1999) report variability in this offset, with a standard deviation of 1.6‰. The authors should rule out that this variability can explain the variability reported in their records, and add some discussion on the factors that can control such an offset (growth rates, shifts in species composition, etc).
Second, I would be caution regarding some of the interpretations of the records. The authors infer an influence of sea ice and modes of atmospheric variability, but overall, I am not convinced that the records fully support these statements. For example, in Fig. 3a, do the different periods highlighted by the grey shaded areas really exhibit statistically distinct chlorophyll d15N values? This is unclear to me and would require a more thorough assessment of variability and trends.
Minor comments:
Introduction: I am not an expert in pigment d15N, but I would have appreciated a more thorough description of how chlorophyll d15N differs from others, such as chlorin d15N (e.g., Higgins et al., 2011, 2010).
Lines 19-20: The authors suggest that elevated d15N values could be explained by increased sea ice and delayed sea-ice melt. However, such conditions are typically associated with stronger light limitation, which would be expected to reduce the degree of nutrient consumption rather than enhance it (i.e., to explain high d15N).
Line 73: I would recommend replacing “phytoplankton pump” with “biological pump,” as export is an ecosystem-level process.
Line 97: Here and throughout the manuscript, greater care should be taken with the use of references. For example, DiFiore et al. (2009) did not investigate wintertime convection, but rather the controls on nitrate d15N in the polar Antarctic Zone. Similarly, on line 106, Sigman et al. (1999) is cited in relation to the typical range of summer MLD in the Antarctic Zone, which is not the focus of this study.
Figure 2: It would be helpful to connect the symbols with lines to make the trends easier to discern.
Lines 204-217: I am not convinced that this discussion of N2 fixation and denitrification is necessary. It is well established that nitrate and organic N d15N in the Southern Ocean are primarily controlled by nitrate assimilation and export (Altabet and Francois, 1994; DiFiore et al., 2009; Fripiat et al., 2019; Sigman et al., 1999). In addition, whether denitrification occurs in the water column or in the sediments has a strong influence on nitrate d15N (Lehmann et al., 2007), and this is not discussed. Finally, the presence of functional genes does not imply that they are expressed or that the associated processes are quantitatively significant.
Lines 223: It is unlikely that penguins exert a significant control on nitrate δ15N (or fixed N) at such remote sites. To the best of my knowledge, their impact has only been demonstrated in the immediate vicinity of penguin colonies. Given the large reservoir of nitrate (and fixed N) in the Antarctic Zone, such localized inputs are likely to be rapidly diluted offshore. At a minimum, the authors should provide references demonstrating a measurable impact at remote locations. In addition, sea ice represents only a minor reservoir of fixed N within the mixed layer. It is therefore unlikely to exert a strong influence on sedimentary d15N. If the authors wish to argue otherwise, this interpretation should be supported with additional references and, ideally, quantitative estimates (i.e., mass and isotopic balance calculation).
Line 239: I think the close agreement between the isotopic offset observed here (between chlorophyll and bulk N d15N) and literature estimates (e.g., Sachs et al., 1999) should be stated more clearly. The similarity appears strong and would help the reader better understand the systematic underlying chlorophyll d15N.
That said, while this correspondence is encouraging, other processes may also influence this offset (e.g., growth rate, shifts in species composition, etc.). I would therefore encourage the authors to discuss these potential effects and assess whether they could account for, or alternatively be ruled out as drivers of, the variability recorded in chlorophyll d15N.
Line 250: Please revise terms such as “highly available 14N.” All 14N in nitrate is available, and the concept of availability is not directly related to isotope fractionation.
On line 274, and throughout this discussion, biogenic silica appears to be treated as a proxy for nitrate uptake, which is not strictly accurate. In contrast, d15N is more directly related to nitrate utilization (e.g., Altabet and Francois, 1994). I would therefore recommend using more cautious and precise wording. This link (biogenic silica and nitrate uptake) should be better described, with a consideration of possible biases (e.g., variable Si:N ratios; Hutchins and Bruland, 1998).
Line 278: “The discrepancy between the chlorophyll d15N record and both MAR signals (Fig. 3a, 3b & 3c) indicates that the degree of nitrate uptake by the regional productivity is unlikely to be the primary driver of chlorophyll d15N variability off Adélie Land.”
This sentence is inconsistent with the established relationship between nitrate d15N (and organic N) and the degree of nitrate consumption (Altabet and Francois, 1994; DiFiore et al., 2009; Fripiat et al., 2019; Sigman et al., 1999). Nitrate remains the ultimate source of N for phytoplankton and must control the mass and isotopic balance of newly produced biomass and their regenerated products. To be a proxy, in my view, chlorophyll d15N should be related to biomass d15N, so the authors should clarify this statement.
In addition, relatively little variation in nutrient consumption can be expected over such timescales, given that nutrient utilization is, on average, very low in the coastal Antarctic Zone. It would therefore be valuable to estimate the extent to which such variability could influence biomass d15N (i.e., the accumulated product in Rayleigh fractionation kinetics; see figure 1 in Sigman and Fripiat, 2019). This aspect warrants further discussion to better assess the sensitivity of chlorophyll d15N to changes in nutrient utilization under nutrient-replete conditions such as those prevailing in the coastal Antarctic Zone.
Discussion in general: While the authors discuss the role of sea ice in driving buoyancy fluxes, they should also consider additional forcings, such as buoyancy input from Antarctic Ice Sheet meltwater (e.g., Bronselaer et al., 2020, 2018). Moreover, factors such as changes in the westerlies and nutrient fertilization are also considered important in explaining variations in sedimentary d15N.
References:
Altabet, M.A., Francois, R., 1994. Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization. Glob. Biogeochem. Cycles 8, 103–116. https://doi.org/10.1029/93GB03396
Bronselaer, B., Russell, J.L., Winton, M., Williams, N.L., Key, R.M., Dunne, J.P., Feely, R.A., Johnson, K.S., Sarmiento, J.L., 2020. Importance of wind and meltwater for observed chemical and physical changes in the Southern Ocean. Nat. Geosci. 13, 35–42. https://doi.org/10.1038/s41561-019-0502-8
Bronselaer, B., Winton, M., Griffies, S.M., Hurlin, W.J., Rodgers, K.B., Sergienko, O.V., Stouffer, R.J., Russell, J.L., 2018. Change in future climate due to Antarctic meltwater. Nature 564, 53–58. https://doi.org/10.1038/s41586-018-0712-z
DiFiore, P.J., Sigman, D.M., Dunbar, R.B., 2009. Upper ocean nitrogen fluxes in the Polar Antarctic Zone: Constraints from the nitrogen and oxygen isotopes of nitrate: POLAR ANTARCTIC NITRATE N AND O ISOTOPES. Geochem. Geophys. Geosystems 10, n/a-n/a. https://doi.org/10.1029/2009GC002468
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Sigman, D.M., Fripiat, F., 2019. Nitrogen Isotopes in the Ocean, in: Encyclopedia of Ocean Sciences. Elsevier, pp. 263–278. https://doi.org/10.1016/B978-0-12-409548-9.11605-7