Exploring the use of compound-specific carbon isotopes as a 1 palaeoproductivity proxy off the coast of Adélie Land , East 2 Antarctica 3

Exploring the use of compound-specific carbon isotopes as a 1 palaeoproductivity proxy off the coast of Adélie Land, East 2 Antarctica 3 Kate Ashley1, Xavier Crosta2, Johan Etourneau2,3, Philippine Campagne2,4, Harry Gilchrist1, 4 Uthmaan Ibraheem1, Sarah Greene1, Sabine Schmidt2, Yvette Eley1, Guillaume Massé4,5 and 5 James Bendle1 6 1School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, 7 B15 2TT, UK 8 2EPOC, UMR-CNRS 5805, Université de Bordeaux, 33615 Pessac, France 9 3EPHE/PSL Research University, 75014 Paris, France 10 4LOCEAN, UMR CNRS/UPCM/IRD/MNHN 7159, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 11 Paris, France 12 5TAKUVIK, UMI 3376 UL/CNRS, Université Laval, 1045 avenue de la Médecine, Quebec City, Quebec, 13 Canada G1V 0A6 14 15 Correspondence to: James Bendle (j.bendle@bham.ac.uk) 16 17

δ13CMeOH was calculated to be ca. -40.8‰ and the δ 13 CFAME values were corrected using: Two hundred and thirty-four samples were taken every 2 cm over the whole core for highly branched     The C19 alkane was used as an internal standard to aid quantification of fatty acid concentrations. However, it 183 should be noted that since this standard was added to samples post-extraction, our concentration estimates are 184 semi-quantitative but can be used to compare concentration changes in different FA compounds. 6 elevated concentrations, broadly decreasing down-core (Fig. 2). Below this, however, two groups clearly 188 diverge. These can be broadly divided into short-chained fatty acids (C16 to C20; SCFAs) and long-chained fatty 189 acids (C22 to C26; LCFAs). Within these groups, the concentrations of different compounds show similar trends, 190 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 196 precursor organism or group of organisms, but the two groups themselves have different producers from each 197 other. These producers may in turn thrive during different seasons or within different habitats and thus, the 198 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 (ca. 1587 -1978 C.E.), to remove correlations 200 associated with preservation changes in the top part of the core (discussed below). Although the R 2 values are 201 not quite as high, they broadly confirm these groupings, with the R 2 values generally being greater within the 202 two groups (n = 73). R 2 values range from 0.93 for the C18 with C20, down to 0.07 for the C18 and C24 (Fig. 4).

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The C18 and C24 FAs are the most abundant compounds within the SCFA and LCFA groups, respectively, and 204 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 205 suggests they are the most likely to be produced by different organisms. Furthermore, these two compounds 206 yielded the highest quality isotope measurements, due to their greater concentrations, clean baseline and 207 minimal coeluting peaks (Fig. S2). Thus, these two compounds (C18 and C24) will be the focus of analysis and 208 discussion.

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Furthermore, Skerratt et al. (1998) compared the FAs produced by P. antarctica and two Antarctic diatoms, in culture samples, and showed that P. antarctica produced a much higher percentage of both saturated FAs (C14-7 abundant source of the saturated C18 FA in the Adélie basin though minor contributions of C18 from other 225 phytoplankton species such as the diatoms and dinoflagellates or even bacteria cannot be excluded (Table S2).   Table S2. Although not all of these sources are likely to be present within the coastal waters 235 offshore Adélie Land, it highlights the wide range of organisms which can produce these compounds, and thus 236 suggests that an autochthonous marine source is likely, especially considering the highly productive nature of

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The complete lack of both unsaturated and short chained (fewer than 16 carbon atoms) FA compounds 254 identified within DTGC2011 samples, even within the top layers, suggests that selective breakdown of 255 compounds has already occurred within the water column and on the sea floor (before burial). Wakeham et al.

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(1984) assessed the loss of FAs with distance during their transport through the water column at a site in the 257 equatorial Atlantic Ocean and estimated that only 0.4 to 2% of total FAs produced in the euphotic zone reached 258 a depth of 389 m, and even less reaching more than 1,000 m depth, the vast majority of material being recycled 259 in the upper water column. Their results also show a significant preference for degradation of both unsaturated 260 and short chained compounds over saturated and longer chain length compounds. Although no studies into the would provide significant opportunity for these compounds to be broken down during transportation through the 263 water column. It is likely, therefore, that the distribution of compounds preserved within the sediments will not 264 be a direct reflection of production in the surface waters, and explains the preference for saturated FAs with 265 carbon chain lengths of 16 and more. It is also possible that some additional production and contribution of FAs

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We compare FA concentrations with other organic compounds (whose source is better constrained) in 282 DTGC2011 to better understand FA sources. Direct comparison between different organic compound classes 283 can be made since both are susceptible to similar processes of diagenesis, in contrast to other proxies such as 284 diatoms. In core DTGC2011, concentrations of di-and tri-unsaturated highly branched isoprenoid (HBI) alkenes (referred to as HBI diene and HBI triene, respectively hereafter) were available.

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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

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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

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The δ 13 C18FA record shows a broadly increasing trend towards more positive values from ca. 1587 until ca. 1920 366 C.E., with short term fluctuations of up to ~4 ‰ superimposed on this long-term trend (Fig. 7). This is followed 367 by a period of higher variability with a full range of 5.6 ‰ until the most recent material (ca. 1999 C.E.), with 368 more negative δ 13 C values between 1921 and 1977 C.E. and a rapid 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 4.6 370 ‰, with a more pronounced negative trend after ca. 1880 C.E. (Fig. 7).

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Below we consider the various factors which may control the carbon isotope value of algal biomarkers produced 372 in the surface waters. Down-core changes in FA δ 13 C are likely to be a function of either the δ 13 C of the dissolved inorganic carbon (DIC) source, changes in the species producing the biomarkers, diagenesis or range of ~1.5 ‰ was identified in water masses between the surface and ~5,500 m depth along a transect from    fractionation factors. Here we measured δ 13 C of individual biomarkers, produced by a more restricted group of observed in our record. Based on concentration data discussed above, it seems that diagenetic overprint is 422 largely complete by ~25 cm (Fig. 2)    spring and summer months during which CO2 is rapidly drawn down and the surface waters become undersaturated. However, upwelling cannot be discarded as a possible contributor to surface water CO2 change.

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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

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Therefore, we suggest that FA δ 13 C signals recorded in DTGC2011 is predominantly a signal of surface water 517 CO2 driven by primary productivity. Indeed, the potential for the δ 13 C of sedimentary lipids to track surface 518 water primary productivity has been recognised in the highly productive Ross Sea polynya. High variability in 519 surface water CO2 values have been measured across the polynya during the summer months (December -520 January), ranging from less than 150 ppm in the western Ross Sea near the coast, to >400 ppm on the northern 521 edge of the polynya. This pattern was closely correlated with diatom abundances, indicating intense drawdown 522 of CO2 in the western region where diatom abundances were highest (Tortell et al., 2011). This spatial variation 523 in productivity is recorded in particulate organic carbon (POC) δ 13 C, and is also tracked in the surface sediments 524 by total organic carbon (TOC) δ 13 C and algal sterol δ 13 C, all of which show significantly higher values in the 525 western Ross Sea. This spatial pattern in sterol δ 13 C was concluded to be directly related to CO2 drawdown at the surface, resulting in average sterol δ 13 C values varying from -27.9‰ in the west, where productivity is greatest, down to -33.5‰ further offshore (Villinski et al., 2008).

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As discussed above, P. antarctica is a likely producer for the C18 FA, a prymnesiophyte algae which has been 557 observed in the Adélie region in summer months residing predominantly along the margin of fast ice, but also 558 further offshore (Riaux-Gobin et al., 2013Vaillancourt et al., 2003). The aversion of F. kerguelensis to 559 sea ice (and thus also the C24 FA producer) in contrast to P. antarctica, may explain the clear lack of coherence 560 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

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Furthermore, δ 13 C18FA shows a broad similarity with Chaetoceros resting spores (CRS) on a centennial scale, 574 with lower productivity at the start of the record, ca. 1587 to 1662 C.E., followed by an increase in both proxies 575 in the middle part of the record, where δ 13 C18FA becomes relatively stable and CRS reaches its highest 576 abundances of the record. This is then followed in the latter part of the record, after ca. 1900 C.E., by both

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The broad similarity to CRS, with lower values recorded during periods of high sea-ice concentrations, suggests 580 that δ 13 C18FA is similarly responding to productivity in stratified water at the ice edge. This supports the 581 hypothesis that δ 13 C18FA is recording primary productivity in the MIZ. Little similarity is evident between the 582 fatty acid isotope records and F. cylindrus and F. rhombica.

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Despite this, we argue that FA δ 13 C has the potential to be used as a productivity proxy, but would be best used 599 in parallel with other environmental proxies such as diatoms abundances or HBIs. Comparison with other proxy 600 data and information from previous studies suggests that the C18 compound may be predominantly produced by