The influence of lateral transport on sedimentary alkenone paleoproxy signals

Alkenone signatures preserved in marine sedimentary records are considered one of the most robust paleothermometers available, and are often used as a proxy for paleoproductivity. However, important gaps remain on the provenance and fate of alkenones, and their impact on derived environmental signals in marine sediments. Here, we analyze 10 the abundance, distribution, and radiocarbon (C) age of alkenones in bulk sediments and corresponding grain-size fractions in surficial sediments from seven continental margin settings in the Pacific and Atlantic Oceans in order to evaluate the impact of organo-mineral associations and hydrodynamic sorting on sedimentary alkenone signals. We find that alkenones preferentially reside within fine-grained mineral fractions of continental margin sediments, with the preponderance of alkenones residing within the fine silt fraction (2-10 μm), and most strongly influencing alkenone C age, and SST signals 15 from bulk sediments as a consequence of their proportional abundance and higher degree of OM protection relative to other fractions. Our results demonstrate that selective association of alkenones with mineral surfaces and associated hydrodynamic mineral sorting processes can alter alkenone signals encoded in marine sediments (C age, content, and distribution) and confound corresponding proxy records (productivity and SST) in the spatial and temporal domain.


Introduction
Since the initial discovery of alkenones (Boon et al., 1978;Volkman et al., 1980), these molecular biomarkers have become one of the most applied and well-established paleoclimate proxies, allowing estimation of sea surface temperature (SST) and primary productivity in most oceanographic settings (Sachs et al., 2000;Raja and Rosell-Melé, 2021). Alkenones are long 25 chain (C37-C39) unsaturated ketones synthesized by some species of haptophytes dwelling in the upper photic zone, most notably the coccolithophore species Emiliania huxleyi and Gephyrocapsa oceanica (Volkman et al., 1980).
The total abundance of C37 alkenones (C37:2 + C37:2) in marine sediments is widely used as a qualitative proxy for primary productivity on the basis that alkenones are a large component of the total carbon of Emiliania huxleyi (Prahl 1988), and that alkenone degradation is not observed upon zooplankton digestion (Volkman et al., 1980;Grice et al., 1998;Grimalt et al., 30 https://doi.org/10.5194/bg-2021-204 Preprint. Discussion started: 2 August 2021 c Author(s) 2021. CC BY 4.0 License. 2000). However, this signal can be altered in marine sediments by the significant loss of alkenones that occurs during their export to and deposition on the seafloor. This "flux attenuation" is site-dependent and generally higher during periods of maximum flux . An additional process that may influence this paleoproductivity indicator includes alkenone input via lateral transport of suspended particles and sediments, which has proven to significantly bias the temperature signal on the Argentine continental margin (Benthien and Müller, 2000) and the Bermuda Rise (Ohkouchi et al., 35 2002). However, a specific determination of the sediment size fraction in which alkenones may preferentially reside is lacking (Sachs et al., 2000). Given the propensity for preferential mobilization and redistribituion of specific grain sizes (McCave et al., 1995;McCave and Hall, 2006a;Pedrosa-Pàmies et al., 2013;Bao et al., 2016) this information is crucial for assessing potential impacts on sedimentary alkenone signals.
The degree of unsaturation of the C37 alkenones, parameterized through the U k' 37 ratio (Eq. 1), varies as a function of the 40 growth temperature of the precursor organisms.
Eq. (1) The relationship between U k' 37 and SST was first quantified in laboratory cultures (Prahl and Wakeham, 1987) with a reported precision of ±0.6°C, leading to the implementation of global calibrations of the U k' 37 ratio from marine surface sediments with instrumental SSTs (Müller et al., 1998;Conte et al., 2006;Tierney and Tingley, 2018). The latter calibration 45 curves exhibit larger associated errors because core-top SST does not always effectively record annual average SST from the overlying water column. In regions like the North Atlantic (>48°N), North Pacific (>45°N), Mediterranean Sea, and the Black Sea, systematic U k' 37-SST decoupling with surface water temperature has been attributed to factors such as seasonal biases in haptophyte productivity and dissolved nutrient concentrations (Epstein et al., 1998), highlighting the need for seasonally-tuned calibrations (Tierney and Tingley, 2018). Selective degradation of the C37:3 due to free radical oxidation 50 and aerobic bacterial processes Rontani et al., 2013) may result in warmer biases in some settings such as SE Alaska, the eastern Pacific, and Santa Monica Basin (Gong and Hollander, 1999;Prahl et al., 2010;Jaeschke et al., 2017). In other regions, such as the Brazil-Malvinas confluence (Benthien and Müller, 2000;Rühlemann and Butzin, 2006), the Nordic and Labrador Seas (Bendle and Rosell-Melé, 2004;Filippova et al., 2016;Tierney and Tingley, 2018) and northern Sargasso Sea [Ohkouchi et al., 2002], marked SST deviations have been attributed to lateral advection of alkenones 55 synthesized in distal regions characterized by distinct surface ocean temperatures. In this regard, the implementation of a general ocean circulation model indicated that long particle residence times and lateral advection of alkenones (via OMmineral interaction) could strongly decouple sediment U k' 37-SST and overlying surface water temperature on continental shelves (Rühlemann and Butzin, 2006). Similarly, advection of pre-aged alkenones associated with mineral surfaces is typically invoked to explain older radiocarbon ages of alkenones in relation to coeval foraminifera in many continental 60 margin and deep ocean settings (e.g., Mollenhauer et al., 2003;Mollenhauer et al., 2005;Kusch et al., 2010;Ausín et al., 2019). Sediments deposited on continental margins are the focus of numerous paleoceanographic studies due to the expanded temporal resolution that they offer over deep-sea sedimentary sequences, they thus dominate global calibration data. Yet, an in-depth investigation of the coupled effects of alkenone-mineral associations and hydrodynamic processes on alkenone-65 based proxy signals recorded in continental margin sediments has not yet been undertaken. Recent studies have highlighted how the interplay between organo-mineral relationships and the grain-size dependent hydrodynamic mineral particle sorting effects exerts strong control on the content and geochemical signatures of OC in continental margin surface sediments (Bao et al., 2018a;Bröder et al., 2018;Magill et al., 2018;Ausín et al., 2021). In general, fine-grained minerals host higher amounts of OM than larger particles by virtue of their higher surface area and hence enhanced physical protection against 70 OM remineralization (Keil et al., 1994b;Mayer, 1994a, b;Hedges and Keil, 1995;Keil and Mayer, 2014). Additionally, the size of mineral particles and their propensity for resuspension largely determines their tendency to be remobilized and dispersed at a given bed shear stress (McCave and Hall, 2006a). Consequently, hydrodynamic particle sorting processes not only selectively translocate OC sorbed to minerals but also expose it to further degradation (Bao et al., 2016;Bao et al., 2018a;Bao et al., 2018b;Ausín et al., 2021). As a component of this OC, alkenones associated with specific grain-size 75 fractions are subject to dispersal and decomposition as a function of the governing hydrodynamic conditions that delineate sediment transport pathways and deposition patterns. Given that the strength and trajectory of mobilizing currents may vary as a function of ocean and climate conditions, and considering continental margins are strategic locations for high-temporalresolution paleoceanographic investigations, greater understanding of the influence of these mechanisms on alkenone signals encoded in marine sediments is needed to improve interpretations of derived proxy records. Here, we explore alkenone-80 mineral grain-size relationships in a suite of surficial sediment samples from seven locations, mostly on productive continental margins, where geochemical evidence exists for the influence of organo-mineral relationships and hydrodynamic particle sorting on OC geochemical signatures and content (Ausín et al., 2021).

Surface sediment samples
Six surface and one near-surface sediment samples were obtained from five different continental margin settings and one deep-ocean sediment drift ( Fig. 1; Table 1). A detailed description of the depositional setting and environmental 90 characteristics of each study site can be found in Ausín et al. (2021). The Peruvian margin site ("PER") is characterized by persistent upwelling that supports very high primary productivity and sustains low oxygen bottom waters (Reimers and Suess, 1983). Sites from Santa Barbara and Santa Monica Basins ("SBB" and "SMB") in the highly productive California margin also feature sub-oxic to anoxic bottom waters favoring OM preservation in underlying sediments. The site abbreviated as "NAT" is from the New England "Mud Patch", a shelf depocenter south of Cape Cod on the Mid-Atlantic 95 Bight that is characterized by moderately high surface ocean productivity and rapid fine-grained deposition (Twichell et al., https://doi.org/10.5194/bg-2021-204 Preprint. Goff et al., 2019). The Namibian margin is characterized by strong upwelling and high primary productivity, and the study site "NAM" is under sporadic influence of high-productivity filaments from the adjacent Lüderitz upwelling cell and is located in an OC depocenter on the upper slope produced by the offshore transport of shelf sediments (Inthorn et al., 2006a).
The deep-ocean site "BER" from the Bermuda Rise in the sub-tropical NW Atlantic is characterized by low primary 100 productivity in overlying surface waters and a fully oxygenated water column. This contourite deposit stems from currents associated with deep-ocean recirculation gyres that result in focused deposition of fine-grained sediment (Laine and Hollister, 1981;Laine et al., 1994). The site named as "NAF", on the NW African margin, is influenced by the Canary Current Upwelling system featuring moderate productivity and bottom water oxygen contents (Zonneveld et al., 2010).
Advective sediment transport has been proposed to explain the relatively low settling rates of coccolithophore calcite plates 105 and alkenones (Fischer et al., 2009), in contrast with the minor or negligible presence of pre-aged alkenones (Mollenhauer et al., 2005).  (Schlitzer, 2021). Acronyms for each site used in the main text are given within brackets. 110 Sediment cores were split onboard every 1 cm and stored at -20°C at the Biogeoscience Group ETH Zurich Sample Repository. For each core, samples from the upper 5 cm (Table 1)

Alkenone extraction and quantification 120
An aliquot of 0.5-30 g of dry sediment from bulk and each grain-size fraction was used for total lipid extraction with MeOH/CH2Cl2 (9:1, v/v) using an EDGE ® automated extraction system. Resulting total lipid extracts were saponified with 0.5M KOH/MeOH prior liquid-liquid extraction of the neutral fraction with hexane. Silica gel column chromatography was applied to separate the neutral fraction into three fractions of increasing polarity (F1 -F3) using hexane, CH2Cl2, and 125 CH2Cl2/MeOH (1:1 v/v), respectively. F2 fractions, containing the alkenones, were analysed by gas chromatography with flame ionization detection (GC-FID) to determine alkenone C37:2 and C37:3 concentrations using n-hexatriacontane as external standard. Corresponding U k' 37 ratios were calculated according to equation (1) by Prahl and Wakeham (1987) and transformed to SST values using the calibration of Tierney and Tingley (2018).

Alkenone radiocarbon analyses
The ketone fractions used for determination of alkenone concentration and unsaturation were further purified for compound specific 14 C analysis following Ohkouchi et al. (2005), with purity of isolated alkenone fractions assessed via GC-FID.
Purified samples were subsequently transferred into tin elemental analyzer (EA) capsules with CH2Cl2 (3 ×50μL). The 135 solvent was removed on a hot plate at 35°C prior wrapping the samples. Blanks were prepared in the same fashion as the samples and spiked with varying masses of oxalic acid II (OXAII; modern 14 C age;  14 C 1.34 ‰) and phthalic anhydride (PHA; infinite 14 C age;  14 C 0 ‰) reference standards in order to quantify and characterize contamination introduced during sample preparation. Samples, spiked blanks, and solvent and capsule blanks were measured within 20 h of preparation as CO2 using an EA system interface coupled to a gas ion source (GIS)-equipped Minicarbon Dating System (MICADAS) 140 McIntyre et al., 2016) at the Laboratory of Ion Beam Physics, ETH Zürich. Data assessment was performed with the BATS data reduction software (Wacker et al., 2010). The model by Hanke et al. (2017)   The fraction-weighted alkenone concentration is comparable to bulk values in PER, NAF, NAM, BER and SMB samples 150 (Table 2), implying a 100-88% alkenone recovery. The large discrepancy between bulk and fraction-weighted alkenone concentrations in SBB and NAT suggests significant loss of alkenones occurred during sediment fractionation in SBB (fraction-weighted values < bulk values) and during manual column chromatography of bulk sediments in NAT (fractionweighted values > bulk values). Alkenone concentrations in bulk sediments are highest in PER (17898 ng gdw -1 ), and decrease in the order NAM > SMB > SBB > NAT > NAF, with minimum values in BER (28 ng gdw -1 ; Fig. 2A and Table 2). 155 With the exception of NAF and BER sediments, where the clay fraction hosts the largest proportion of alkenones followed by FS, alkenone concentrations are highest within the FS fraction at all sites. Alkenone concentrations normalized to OC% also show that the OC in the smallest grain sizes (FS and Clay) are associated with the highest alkenone abundances (Fig.   2B). The relative proportion of the di-and tri-unsaturated alkenones exhibit significant variability among grain size fractions at
Alkenone ages vary among sites, ranging from 2300 14 C yr in NAM to 500 14 C yr in PER. Comparison of these results with bulk OC and planktic foraminifera 14 C ages from the same samples (Ausín et al., 2021) shows alkenones and OC ages are comparable, and both are older than corresponding planktic foraminifera 14 C ages.  Purification of alkenones for radiocarbon dating was possible in some of the size fractions for these four samples. Alkenones contained in sand fractions are the oldest, while those hosted within FS and CS show the smallest age offsets with respect to bulk sediments (Fig. 4).   Only SST from samples from SBB and SMB are comparable to atlas data, whereas temperature differences ranging from -6°C to 3°C are observed at the other sites (Fig. 5B). Abundance-weighted average SST of the analyzed grain-size fractions compares relatively well with bulk SST except at BER (Table 2), which shows a -4.4°C difference. The latter is attributed to the lack of detectable alkenones in the sand fraction of BER. Except for BER, SST discrepancies imply core-top SST is warmer than surface water temperature. U k' 37-SST shows significant 205 variability among size grain size fractions at each site (Fig. 6A). The smallest SST variation among size fractions is observed at PER, SMB and NAM. Sand shows the warmest temperature signal in relation to other fractions at 5 out of 6 locations ( Fig. 6). Overall, FS shows the smallest temperature offsets with bulk sediment (Fig. 6B). No specific fraction shows larger/smaller offsets with annual averaged SST (Fig. 6C). 210 Figure 6. SST at each site A) Bulk-, grain-size, and annual mean SST (Locarnini et al., 2019). Temperature difference between B) bulk-and annual mean SST and C) each grain-size fraction and bulk-SST.

Alkenone signals (and biases) from bulk sediment samples
Alkenone concentrations in bulk sediments follow the identical pattern to that of OC% (R 2 =0.99, n=7), indicating similar preservation mechanisms for both and that bulk OC is predominantly derived from marine primary production at each 220 location (Fig. 7A). These results support the hypothesis that alkenone fate in marine sediments is largely influenced by organo-mineral relationships and hydrodynamic mechanisms (Ausín et al., 2021).
Older-than-foraminifera alkenone ages indicate contributions of pre-aged alkenones in the four samples analyzed (Fig. 3), and previously observed at the three other studied regions: Santa Barbara Basin, Bermuda Rise, and, to a lesser extent, NW African margin (Ohkouchi et al., 2002;Mollenhauer and Eglinton, 2007). These results imply that alkenone signatures are 225 influenced by processes such as bioturbation, preferential degradation of fresh alkenones, and/or translocation of older alkenones (e.g., lateral advection via entrainment in sediment resuspension-deposition cycles or nepheloid layers) associated with along-or across-margin transport. A significant influence from bioturbation is unlikely since all sites are characterized by high sedimentation rates (>20 cm/kyr) (Bothner et al., 1981;Wefer et al., 1990;Schaaf and Thurow, 1995;Ohkouchi et al., 2002;Mollenhauer et al., 2005;Inthorn et al., 2006b;Balestra et al., 2018), and given that some sites (e.g., SMB) contain 230 varved sediments deposited under the influence of anoxic or sub-oxic bottom waters. In contrast, prolonged particle aging due to resuspension and downslope transport is a feature of OC-rich continental margin sediments (Mollenhauer et al., 2008). The joint assessment of 14 C ages and SSTs among grain-size sediment fractions at each site provides insights into the influence of selective degradation and alkenone translocation mechanisms (Secs. 4.3. and 4.4).
Older alkenones may carry a different temperature signal than that of the water column overlying the depositional site if they 235 originate from a distal location or were synthesized during colder/warmer past periods. Alkenones from SMB and SBB are found to accurately reflect local instrumental SST (Fig. 5), while a +1-3°C discrepancy (towards warmer temperatures) is observed at other locations with the exception of BER (-6 °C). In both cases, these temperature discrepancies exceed the analytical uncertainty. Such a warmer bias is a common feature of sediments from many locations with the exception of those underlying tropical waters (Conte et al., 2006;Prahl et al., 2010). Previous authors argue that this bias cannot be solely 240 explained by faster degradation of the more-unsaturated C37:3 alkenone (Rosell-Melé et al., 1995) and ascribe it to seasonal production and/or lateral transport of alkenones (Goñi et al., 2001;Sachs and Anderson, 2003;Conte et al., 2006). With respect to BER, we used sub-surface sediments (2-5 cm; foraminifera 14 C age=900±50 yr), but alkenone-SST from the coretop (0-1 cm) of the exact same core (Ohkouchi et al., 2002) leads to a -6.6°C (cold) bias. Recent evidence on the advection of lithogenic particles from the shelf of the NE Canadian maritime provinces (Nova Scotia, Newfoundland) supports lateral 245 transport of alkenones from these colder and more productive waters, previously proposed to explain hydrogen isotope and 14 C-depleted values of alkenones at this site (Ohkouchi et al., 2002;Englebrecht and Sachs, 2005;Hwang et al., 2021), and consistent with the cold bias found at BER.
In light of the strong agreement between OC% and alkenone concentration and the temporal and temperature biases observed in bulk surface sediments from all the studied sites, we speculate that alkenone-proxy signals from continental margin 250 https://doi.org/10.5194/bg-2021-204 Preprint. Discussion started: 2 August 2021 c Author(s) 2021. CC BY 4.0 License. sediments can be strongly modulated by the interplay between organo-mineral relationships and differential hydrodynamic sorting of mineral particle sizes. Alkenone concentrations, 14 C ages and U k' 37-SST values measured on specific grain-size fractions provide a means to evaluate this hypothesis.

Influence of hydrodynamic sorting processes on sedimentary alkenone signals 255
Despite evidence of substantial alkenone loss during sample workup in one or several size fractions from SBB and bulk sediments from NAT, the overall strong positive correlation between alkenone concentration and OC% in sediment fractions (R 2 =0.81) indicates mutual preservation mechanisms also exist within mineral grain size classes (Fig. 7A). Hence, and as observed for OC (Ausín et al., 2021), the large differences in alkenone concentration among grain size fractions correspond 260 to preferential association with, and protection by mineral grains having greater surface area (i.e., FS) (Premuzic et al., 1982;Keil et al., 1994a;Keil et al., 1994b) and to further exposure to degradation for alkenones (and associated organic matter) residing in the least-cohesive grain size fraction that is more prone to resuspension (i.e., CS) (McCave et al., 1995;McCave and Hall, 2006b).
When alkenone concentrations in size fractions are normalized to the bulk sediment mass, the primary contribution of FS -265 and CS to a lesser extentis apparent (Fig. 7B). Given the propensity of FS to resuspension and mobilization under strong currents (McCave and Hall, 2006b), we argue that the temporal offsets and temperature biases observed in bulk sediments can be largely ascribed to the lateral supply of pre-aged/allochthonous alkenones sorbed onto the surfaces of fine-grained, mobilizable (fine-silt) minerals. To a lesser extent, advection of coarser grains (i.e., CS) can also contribute significantly to signals embedded in bulk sediments. Consistent with this notion, FS shows the smallest age and temperature offset with 270 respect to corresponding bulk sediments ( Fig. 4B and 6B). In addition to SST and temporal offsets, our results suggest that the alkenone-based productivity proxy (Raja and Rosell-Melé, 2021) may also be influenced by the translocation and deposition of fine sediments from distal regions. Its impact can be particularly relevant in regions where the contribution of silt minerals to the bulk sediment mass is significant.

Selective alkenone degradation during lateral particle transport 280
The strong grain-size dependence of OC-14 C ages found in all the study sites (Ausín et al., 2021) is not uniformly observed for the more limited alkenone-14 C age data set (Fig. 4). Yet, the strong positive linear relationship observed between both (R 2 =0.78) suggests alkenones could exhibit a similar age-grain-size relationship driven by the differential influence of hydrodynamic processes on mineral grain sizes (Fig. 8). Overall, FS and sand show warmer-than-instrumental alkenone-285 derived SSTs at most sites (Fig. 6C), with the sand fraction exhibiting the greatest warm bias and oldest ages. These results may reflect extensive diagenetic alteration as a consequence of two non-exclusive mechanisms: i) input of pre-aged alkenones synthesized in warmer waters, or ii) selective microbial or abiotic oxidative degradation of more labile (fresher) polyunsaturated (C37:3) alkenones as a consequence of decreasing mineral protection with increasing grain size. Although there has been evidence for (Gong and Hollander, 1997;Hoefs et al., 1998) and against (Sikes et al., 1991;Teece et al., 290 1998;Grimalt et al., 2000) the impact of selective alkenone degradation on sediment U k' 37 ratios, more recent work has demonstrated that autoxidation and aerobic bacterial degradation can cause selective degradation of more unsaturated alkenones, altering corresponding U k' 37 ratios, resulting in warm temperature biases of up to 5.9 °C (Rontani et al., 2013, and references therein). Given that the relative increase of SST and 14 C age is most pronounced for the [comparatively immobile] sand fraction, and that it is difficult to envision how advected alkenones systematically carry a warmer signal than that of the 295 overlying water column for diverse locations, we suggest that selective degradation of C37:3 provides the most viable explanation. While further evidence is required in order to attribute warmer biases to selective degradation of C37:3 within specific size fractions, a universal SST-grain size relationship is not expected because corresponding SST depends on the temperature of surface waters where alkenones were produced. In this context, a much colder initial surface ocean signal than that at the depositional location could mask the influence of selective degradation during oxic transport.

Site-specific hydrodynamic mechanisms 305
Alkenone ages from all PER grain size fractions are similar and close to modern values (Fig. 4). This observation, along with the largest alkenone concentrations, are attributed to the high vertical flux of fresh OM (Reimers and Suess, 1983) and agree with a minor, although discernable, effect of hydrodynamic sorting on OC signals (Ausín et al., 2021). Alkenone-derived SST values of size fractions and bulk sediments at PER are similar, but differ markedly from (are 2.3°C warmer than) 310 instrumental SST (Fig. 6) Kienast et al., 2012). Resuspension of recently deposited bottom sediments from the shelf and offshore transport as suggested by Pak et al. (1980) to explain particle advection maxima at 140-200 m water depth at this location is discarded because across-shelf transport would hypothetically translocate a colder signal.
Selective degradation of C37:3 seems unlikely, as this is expected to exert a differential impact on the U k´3 7 of the different fractions based on their size (i.e., propensity for resuspension and OM exposure to oxic conditions during transport) and 315 mineral surface area (i.e., potential for OM protection), whereas our results show comparable U k´3 7 for all fractions. Prior authors (Rein et al., 2005;Kienast et al., 2012) speculated sedimentary C37 alkenones at this location are skewed towards El Niño events arguing coccolithophores preferentially grow in oligotrophic waters. In fact, while coccolithophores generally dominate the phytoplankton community in oligotrophic waters their absolute abundance is highest in high-nutrient periods/regimes (e.g., Flores and Sierro, 2007). Alkenone producers E. huxleyi and G. oceanica are generally linked to 320 eutrophic waters and periods of maximum primary productivity (Tyrrell and Merico, 2004). Recent work reveals a significant positive correlation between C37 alkenone concentrations from a global surface sediment compilation and maxima https://doi.org/10.5194/bg-2021-204 Preprint. Discussion started: 2 August 2021 c Author(s) 2021. CC BY 4.0 License.
Chla in overlying waters (Raja and Rosell-Melé, 2021). Accordingly, we suggest preferential alkenone production during the austral summer , when surface waters are warmest and primary productivity is at its highest, is the most feasible explanation for the warm bias from sedimentary alkenones observed at this location. 325 Alkenone ages from grain-size fractions at NAM are more dissimilar than at PER, and are 2000-3000 14 C yr older than coeval foraminifera. Moreover, alkenone-derived SST values among grain size fractions range from 15.8 °C to 17.9 °C ( Figs. 4 and 6). Both sites are characterized by high productivity, low oxygen exposure and local deposition, and defined as "initial" depositional systems (Ausín et al., 2021) in terms of OC dispersal and deposition. Site-specific characteristics, such as lower primary productivity and a broader shelf might favor a larger impact of hydrodynamic processes on sedimentary 330 OC and alkenone signals at NAM. Our results suggest lateral supply of pre-aged alkenones influenced by hydrodynamic particle sorting and potentially originating from different locations on the margin, and are consistent with prior models of sediment transport by bottom and intermediate nepheloid layers leading to the formation of an upper slope OC depocenter (Inthorn et al., 2006a;Inthorn et al., 2006b). However, bulk SST only differs 0.8°C from annual averaged SST, indicating that the apparent influence of hydrodynamic mineral sorting on sedimentary alkenone 14 C age and abundance might not 335 necessarily impart an equivalent bias in alkenone temperature signals. Past changes in the temperature gradient between the sites of alkenone production and deposition may, however, lead to larger and unnoticed SST biases in the sedimentary record.
Large alkenone age and temperature discrepancies among grain-size fractions are observed in NAT (Fig. 4). NAT is located within the New England Mud Patch, where large amounts of FS advected by strong bottom currents and storm-induced 340 transport of sand occurs (Goff et al., 2019), enhancing the oxygen exposure time of OM associated with both grain size fractions during transport. This mechanism would foster alkenone aging/input of pre-aged alkenones as well as selective degradation of C37:3 in low-surface-area minerals, as observed for sand fractions. Sedimentary alkenones reflect a warmer signal than that of the overlying surface water, in contrast with the colder bias observed from offshore, slope sediments (Hwang et al., 2014) explained by lateral advection of resuspended sediments from a colder upstream location (Hwang et al., 345 2009;Hwang et al., 2021). On the shelf (< 150 mwd), however, accumulation of advected fine silt sediments occurs under a west-directed transport, as shown by seismic profiles and the presence of active southwestward megaripples (Twichell et al., 1981;Goff et al., 2019). Lateral transport of fine sediments to this site from the Georges Bank to the east, as hypothesized by Twichell et al. (1981), would result in the entrainment and deposition of sedimentary alkenones carrying a warmer signal.
Large alkenone age offsets are also apparent among grain-size fractions from SMB. In this basin, the impact of 350 hydrodynamic processes is strongly modulated by basin topography and by local variability in bottom water oxygen content, which can lead to differences in alkenone ages of flank and depocenter sediments (Mollenhauer and Eglinton, 2007). The fidelity of the sediment-SST signature in SMB may be coincidental, as the presence of aged alkenones in all size fractions indicates addition of allochthonous (advected) material. Indeed, bomb radiocarbon was present in co-eval planktic foraminifera, whereas this was not detectable in corresponding alkenone samples. These results may imply rapid degradation 355 of fresh alkenones and/or alkenone input from distal locations. https://doi.org/10.5194/bg-2021-204 Preprint. Discussion started: 2 August 2021 c Author(s) 2021. CC BY 4.0 License.

Conclusions
Alkenone concentration, 14 C age and SST was determined in surficial sediments and corresponding grain-size fractions (clay, 360 fine and coarse silt, and sand) retrieved from 6 continental margin settings.
Our results provide clear evidence for alkenone transport as a consequence of their intimate association with surfaces of finegrained minerals; subsequent hydrodynamic mineral sorting and associated exposure to oxic degradation during transport imparts a strong influence on sedimentary alkenone signals. Alkenones preferentially reside within the fine silt fraction (2-10 µm) of sediments. Overall, this fraction is the largest alkenone contributor to marine sedimentary signals and exerts a 365 predominant control on the alkenone concentration, 14 C age and derived SST values manifested in bulk sediments. Alkenone 14 C ages from FS (but also CS) indicate resuspension and protracted transport of alkenones from distant regions (or past time periods), suggesting that the intimate association of alkenones with fine grained sediments has important implications for the paleoreconstruction of primary productivity and SST based on alkenone concentrations and distributions.

Significant U k'
37-SST variability is observed among grain size fractions. We suggest that the predominantly warmer-than-370 instrumental SST may reflect two alternative processes: 1) selective degradation of the tri-unsaturated C37 alkenone attributed due to lower OM protection offered by larger particles under oxic conditions, and 2) systematic input of allochthonous alkenones synthesized in warmer waters. Further work is needed to determine the validity and importance of these scenarios.
Assessment of alkenone amount, 14 C age, and SST in grain-size fractions sheds important new light on processes controlling 375 alkenone signatures in bulk sediments from the studied sites, including vertical settling of fresh material, lateral transport of allochthonous and pre-aged alkenones and alkenone degradation. The combined influence of alkenone-mineral associations and hydrodynamic particle sorting processes on sedimentary alkenone signals is discernable at all sites, ranging from almost negligible (e.g., at PER) to substantial (e.g., BER). Yet, pronounced impacts on alkenone 14 C age and concentration do not necessarily impart an equivalent bias in U k' 37-SST (e.g., SBM and NAM), as the latter also depends on the temperature 380 gradient between the sites (or time periods) of alkenone production and deposition. Past changes in this temperature gradient could, however, lead to larger SST biases in the sedimentary record.
Our results highlight the importance of considering the influence of hydrodynamic processes (e.g., lateral transport) on sedimentary alkenone signatures (amount, age, and temperature) and their relationship to surface waters overlying the depositional location. 385 Data availability. All original data used in this study, necessary to understand, evaluate, and replicate this research, are presented and available in tables within the main text.