Point-by-point response to comments Title: Fatty acid carbon isotopes: a new indicator of marine Antarctic paleoproductivity?

Minor formatting and technical corrections remain: Line 52 – I read the rebuttal and the revision, ‘freely extractable (using a standard solvent extraction protocol)’ remains is obscure as yields differ between common solvent extraction methods (sonication, microwave, soxhlet and pressurized ASE or EDGE extraction). The method can be detailed in the methods. Here, suggest “free (solvent-extractable)” would suffice. Amended as suggested

with N2 at 35°C, the total lipid extract was fractionated over a silica column into an apolar and a polar fraction 104 using 3 mL hexane and 6 mL CH2Cl2/MeOH (1:1, v:v), respectively. HBIs were obtained from the apolar 105 fraction by the fractionation over a silica column using hexane as eluent following the procedures reported by One hundred and eighteen samples were taken every 4 cm over the whole core for diatom analyses. Sediment 116 processing and slide preparation followed the method described in Crosta et al. (2020).

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Diatom counting followed the rules described in Crosta and Koç (2007). Around 350 diatom valves were 118 counted in each sample at a 1000X magnification on a Nikon Eclipse 80i phase contrast microscope. Diatoms 119 were identified to species or species group level. Absolute abundances of diatoms were calculated following the 120 equation detailed in Crosta et al. (2008). The relative abundance of each species was determined as the fraction 121 of diatom species against total diatom abundance in the sample.   (Fig. 4). Below this, however, two groups clearly 132 diverge. These can be broadly divided into short-chained fatty acids (C16 to C20; SCFAs) and long-chained fatty 133 acids (C22 to C26; LCFAs). Within these groups, the concentrations of different compounds show similar trends, 134 but the two groups (SCFAs vs LCFAs) show different trends to each other (Gilchrist, 2018). This is confirmed

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These distinct groupings suggest that compounds within each group (SCFAs and LCFAs) likely have a common 140 precursor organism or group of organisms, but the two groups themselves have different producers from each 141 other. These producers may in turn thrive during different seasons or within different habitats and thus, the 142 isotopic composition of compounds from these different groups may record different environmental signals.

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R 2 values were also calculated for samples below 25 cm only, to remove correlations associated with 144 preservation changes in the top part of the core (discussed below). Although the R 2 values are not quite as high, 145 they broadly confirm these groupings, with the R 2 values generally being greater within the two groups (n = 73).

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R 2 values range from 0.93 for the C18 with C20, down to 0.07 for the C18 and C24 (Fig. 3).

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The C18 and C24 FAs are the most abundant compounds within the SCFA and LCFA groups, respectively, and 148 also the least correlated with each other both in the whole core (R 2 = 0.5) and below 25 cm (R 2 = 0.07), which 149 suggests they are the most likely to be produced by different organisms. Furthermore, these two compounds 150 yielded the highest quality isotope measurements, due to their greater concentrations, clean baseline and 151 https://doi.org/10.5194/bg-2020-124 Preprint. Discussion started: 23 April 2020 c Author(s) 2020. CC BY 4.0 License. minimal coeluting peaks (Fig. S2). Thus, these two compounds (C18 and C24) will be the focus of analysis and 152 discussion. values and those reported in the literature for P. antarctica (Kopczynska et al., 1995;Wong and Sackett, 1978).

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Unfortunately, the absence of P. antarctica in sediments, as it does not biomineralize any test, precludes the   and Dinophyceae produce the highest proportions of the saturated C18 FA, the former to which P. antarctica 167 belongs. They also showed that the majority of FAs produced were the unsaturated form which are 168 preferentially broken down in the water column and sediments. As such, although the C18 FA represents only a 169 small proportion of the total FA fraction, its higher preservation rate increases its proportion in the sediment.

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Chaetoceros simplex and Odontella weissflogii, from culture samples. Of the FAs produced by P. antarctica, 177 52% were saturated FAs (C14-C20) compared to just 14 and 11% for the two diatoms, respectively, the latter 178 instead producing much more of the mono-and polyunsaturated FAs. The percentage of C18 FA produced by P.

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antarctica was also 4.1 and 12.5 times greater than the percentage of C18 produced by C. simplex and O.    Table S2. Although not all of these sources are likely to be present within the coastal waters 192 offshore Adélie Land, it highlights the wide range of organisms which can produce these compounds, and thus 193 suggests that an autochthonous marine source is entirely possible, especially considering the highly productive      would provide significant opportunity for these compounds to be broken down during transportation through the 220 water column. It is likely, therefore, that the distribution of compounds preserved within the sediments will not 221 be a direct reflection of production in the surface waters, and explains the preference for saturated FAs with

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We compare FA concentrations with other organic compounds (whose source is better constrained) in 238 DTGC2011 to better understand FA sources. Direct comparison between different organic compound classes 239 can be made since both are susceptible to similar processes of diagenesis, in contrast to other proxies such as 240 diatoms. In core DTGC2011, concentrations of di-and tri-unsaturated highly branched isoprenoid (HBI) alkenes 241 (referred to as HBI diene and HBI triene, respectively hereafter) were available.

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One key similarity between both the HBI diene and triene, and the FA concentrations is that the highest

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The C24 FA record also shows some similarity with the HBI triene record. This appears to be mostly in the top 288 part of the core where the highest concentrations are found. The reason for this resemblance is unclear, 289 especially considering the lack of correlation between the C24 and C18 FA concentrations. However, it may relate 290 to the better preservation in younger samples. The weaker coherence between the C24 and the HBI triene, and 291 also HBI diene, suggests that the C24 FA is predominantly produced by an organism which is not associated with  Down-core changes in δ 13 C for the C18 and C24 FAs (δ 13 C18FA and δ 13 C24FA, respectively) ( Fig. 6 and 7) clearly 305 show different trends, with very little similarity between them (R 2 = 0.016). This further supports the idea that 306 these compounds are being produced by different organisms, and thus are recording different information. 315 ‰) to be 6.7 ‰ higher than pelagic phytoplankton (-30.7 ‰) from the same region.

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The higher δ 13 C of the C18 FA could therefore be indicative of P. antarctica living partly within the sea ice, e.g.

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during early spring before ice break up. The more negative δ 13 C24FA suggests it is more likely to be produced by  The δ 13 C18FA record shows a broadly increasing trend towards more positive values from ca. 1587 until ca. 1920 321 C.E., with short term fluctuations of up to ~4 ‰ superimposed on this long-term trend (Fig. 7). This is followed 322 by a period of higher variability with a full range of 5.6 ‰ until the most recent material (ca. 1999 C.E.), with 323 more negative δ 13 C values between 1921 and 1977 C.E. and rapid a shift toward more positive values thereafter.

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In contrast, the δ 13 C24FA record overall shows a weak, negative trend, with large decadal fluctuations of up to 4.6 325 ‰, with a more pronounced negative trend after ca. 1880 C.E. (Fig. 6 and 7).     :0 and as much as 6.9‰ in the C18:1, under anoxic conditions. This suggests that diagenesis could affect FA 371 δ 13 C in core DTGC2011. However, these observed changes are rapid (days to months), occurring on timescales 372 which are unresolvable in the FA δ 13 C record (annual to decadal), and thus may have no effect on the trends 373 observed in our record. Based on concentration data discussed above, it seems that diagenetic overprint is 374 largely complete by ~25 cm (Fig. 4). In the top 25 cm of the core, the δ 13 C24FA values increase by ~2.5 ‰,       However, the core site is in a relatively sheltered area and is probably not affected by significant upwelling.

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Based on these studies, changes in atmospheric CO2 concentration and δ 13 C of the source appear to be unlikely  Given that changes in atmospheric CO2, source signal, sea ice algae or diagenesis seem unable to explain the 454 full range of variability seen in the FA δ 13 C record, the most plausible driver appears to be changes in surface

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Therefore, we suggest that FA δ 13 C signals recorded in DTGC2011 is predominantly a signal of surface water 466 CO2 driven by primary productivity. Indeed, the potential for the δ 13 C of sedimentary lipids to track surface 467 water primary productivity has been recognised in the highly productive Ross Sea polynya. High variability in western Ross Sea. This spatial pattern in sterol δ 13 C was concluded to be directly related to CO2 drawdown at 475 the surface, resulting in average sterol δ 13 C values varying from -27.9‰ in the west, where productivity is 476 greatest, down to -33.5‰ further offshore (Villinski et al., 2008).

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A similar relationship is evident in Prydz Bay, where POC δ 13 C was found to be positively correlated with POC

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This suggests it is possible to apply FA δ 13 C as a palaeoproductivity indicator in the highly productive Adélie 481 polynya environment. However, it is important to constrain the most likely season and habitat being represented, 482 since phytoplankton assemblages vary both spatially (e.g. ice edge or open water) and temporally (e.g. spring or 483 summer). The incredibly high sedimentation rate (1-2 cm yr -1 ) within the Adélie Basin is thought to result, on 484 top of regional high productivity, from syndepositional focusing processes bringing biogenic debris from the  Comparison of down-core variations in FA δ 13 C with other proxy data can also be used to decipher the main 495 signal recorded. Comparison between δ 13 C24FA and the major diatom species abundances within the core shows 496 a reasonably close coherence with Fragilariopsis kerguelensis, particularly since ~1800 C.E. (Fig. 6). in the down-core trends in δ 13 C18FA and δ 13 C24FA (Fig. 7). Thus, we hypothesise that δ 13 C18FA is recording surface 509 water CO2 driven by productivity in the MIZ, whilst δ 13 C24FA is recording surface water CO2 in more open 510 water, further from the sea-ice edge.

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HBI diene concentrations indicate elevated fast ice cover between ~1919 and 1970 C.E., with a particular peak 512 between 1942 and 1970 C.E., after which concentrations rapidly decline and remain low until the top of the core 513 (Fig. 7). Abundances of F. curta, used as a sea-ice proxy, similarly show peaks at this time indicate increased 514 sea-ice concentration (Campagne, 2015) (Fig. 7). δ 13 C18FA indicates a period of low productivity between ~1922 515 and 1977 C.E., broadly overlapping with this period of elevated fast ice concentration (Fig. 7), with a mean 516 value of -27.12‰. This is compared to the mean value of -26.23‰ in the subsequent period (~1978 to 1998 517 C.E.) during which HBI diene concentration remain low (Fig. 7). This suggests that productivity in the coastal

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Furthermore, δ 13 C18FA shows a broad similarity with Chaetoceros resting spores (CRS) on a centennial scale, 522 with lower productivity at the start of the record, ca. 1587 to 1662 C.E., followed by an increase in both proxies 523 in the middle part of the record, where δ 13 C18FA becomes relatively stable and CRS reaches its highest 524 abundances of the record. This is then followed in the latter part of the record, after ca. 1900 C.E., by both 525 proxies displaying lower values overall. CRS are associated with high nutrient levels and surface water 526 stratification along the edge of receding sea ice, often following high productivity events (Crosta et al., 2008).

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The broad similarity to CRS, with lower values recorded during periods of high sea-ice concentrations, suggests 528 https://doi.org/10.5194/bg-2020-124 Preprint. Discussion started: 23 April 2020 c Author(s) 2020. CC BY 4.0 License. that δ 13 C18FA is similarly responding to productivity in stratified water at the ice edge. This supports the 529 hypothesis that δ 13 C18FA is recording primary productivity in the MIZ.

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Comparison with other proxy data and information from previous studies suggests that the C18 compound may