Nutrient utilization and diatom productivity changes in the low-latitude SE Atlantic over the past 70 kyr: Response to Southern Ocean leakage

Eastern Boundary Upwellings (EBUs) are some of the key loci of biogenic silica (opal) burial in the modern ocean, representing important productive coastal systems that extraordinarily contribute to marine organic carbon fixation. The Benguela Upwelling System (BUS), in the low-latitude SE Atlantic, is one of the major EBUs, which is under the direct influence of nutrient-rich Southern Ocean waters. Quantification of past changes in diatom productivity through time, in response to Late Quaternary climatic change, feeds into our understanding of the sensitivity of EBUs to future climatic 5 perturbations. Existing sediment archives of silica cycling include: opal burial fluxes, diatom assemblages and opaline silicon isotopic variations (denoted by δSi). Burial fluxes and siliceous assemblages are limited to recording the remains reaching the sediment (i.e. export), and δSi variations are complicated by species-specific influences and seasonality. Here, we present the first species-specific δSi record from the BUS, encompassing full glacial conditions to the Holocene. In addition to export, our new data allows us to reconstruct utilisation of dissolved Si in surface waters in an area with strong input from Southern 10 Ocean waters. Our new archives show that there was enhanced upwelling of Southern Ocean Si-rich water, and accompanied strong silicic acid utilisation by coastal dwelling diatoms, during Marine Isotope Stage 3 (60-40 kyr). This pulse of strong silicic acid utilisation was followed by a weakening of upwelling and coastal diatom Si utilisation into MIS2, before an increase in pelagic diatom Si utilisation across the deglaciation. We combine our findings with mass balance model experiments to show that changes in surface water silica cycling through time are a function of both upwelling intensity and utilisation changes, 15 illustrating the sensitivity of EBUs to climatic change on glacial-interglacial scales.

Benguela Upwelling System (BUS; Fig. 1) in the SE Atlantic is one of the major EBUs, and is the under the influence of DSi-rich SO waters (Berger and Wefer, 1996;Berger et al., 2002). Quantification of BUS Si production in the Late Quaternary is important for understanding the functioning of this highly-productive EBU, and the sensitivity of the strong and dynamic 25 biological production to changes in wind-driven mixing and SO leakage and ventilation (Hendry and Brzezinski, 2014). The silicic acid leakage hypothesis (SALH) postulates that shifts in the leakage (i.e. export) of DSi, relative to other nutrients, within Antarctic Intermediate Water (AAIW; Fig. 1) could occur on glacial-interglacial timescales as a result of changes in SO diatom physiology and silicification due to alleviation of iron stress from dust supply (Brzezinski et al., 2002). The northward supply of waters with a higher DSi-to-nitrate ratio via AAIW during full glacials would promote low latitude diatom growth 30 at the expense of other non-siliceous phytoplankton, resulting in a weakening of the carbonate pump, changing seawater alkalinity and contributing to atmospheric pCO 2 drawdown (Matsumoto and Sarmiento, 2008). Opal burial and geochemical archives show that DSi leakage during the last glacial maximum (LGM) is variable, with some evidence for this mechanism in the South Pacific (Chase et al., 2003;Rousseau et al., 2016), but patchy opal burial response in the Equatorial Pacific (Kienast et al., 2006). In the Atlantic, there is stronger evidence that DSi leakage and opal production was higher during the 35 deglacial rather than the LGM (Hendry et al., 2016;Meckler et al., 2013). The impact of DSi leakage on opal burial during the deglacial is heterogeneous, both in the Atlantic and Pacific (Bradtmiller et al., 2006(Bradtmiller et al., , 2007Dubois et al., 2010), suggesting that ventilation of DSi-rich waters is required to promote diatom growth (Hendry and Brzezinski, 2014). Fewer archives exist for Late Quaternary DSi leakage under full glacial conditions, although one record highlights potential SO leakage and increase in AAIW DSi in Marine Isotope Stages (MIS) 3/4 in the tropical NW Atlantic (Griffiths et al., 2013). Analogous to carbon 40 cycling, siliceous production can be considered as two interconnected processes: net utilisation is the proportion of DSi taken up by diatoms to form opal in ocean surface waters, and export production is the opal that sinks out of the surface waters into the deep (Ragueneau et al., 2000). However, when reconstructing marine siliceous primary production, the only evidence available is the opal buried as siliceous remains of microorganisms within marine sediments. Understanding the link between surface DSi utilisation, recycling, and export of opal is important to understand ecosystem function and organic matter sequestration.

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The stable Si isotopic composition (denoted in per mil relative to standard NBS28/RM8546 by δ 29 Si or more commonly δ 30 Si, Equ. 1, 2) of opal provides an archive of DSi utilisation, given the preferential uptake of lighter Si isotopes during diatom biomineralisation (De La Rocha et al., 1997).
Culture studies and field observations indicate that diatoms discriminate Si isotopes by a fractionation factor, e, of approximately -1.1 ‰, although estimates for this value range from -0.4 to -2.5 ‰ (Hendry and Brzezinski, 2014, & references therein). The application of this utilisation proxy is complicated by several unknowns: changes in the initial isotopic compo-55 sition of ambient seawater through time (Horn et al., 2011); potential seasonal and ecological bias (Swann et al., 2017); and potential species-specific fractionation of Si isotopes in mixed assemblages (Sutton et al., 2013). Here, we present the first late Quaternary δ 30 Si records from the BUS during the full glacial conditions of Marine Isotope Stage (MIS) 4 to the late Holocene, overcoming these challenges by using only two large centric species from a well-documented sediment core. We will use these records, together with a simple box model, to reconstruct past changes in seawater composition and DSi supply through time.

Material and Methods
Gravity core GeoB3606-1 was collected in the SE Atlantic Ocean (BUS, Fig. 1, 25 • 28 S, 13 • 05 E, 1785 m water depth), providing an exceptionally high-resolution paleoclimatic archive of nutrient conditions, productivity, and sea surface temperature (SST) variations off Namibia for the past 70 kyr (McKay et al., 2016;Romero et al., 2015Romero et al., , 2003Romero, 2010;Shukla and Romero, 2018, Supplemental Material). The robust age model for core GeoB3606-1 has been published elsewhere (Romero,65 2010; Romero et al., 2015), with additional radiocarbon dates from McKay et al. (2016). The published conventional 14 C ages have been converted to calendar ages. High-resolution counts of diatom valves, bulk biogenic components, planktonic and benthic foraminifera stable isotopes and alkenone-based SST have been previously published (Romero, 2010;Romero et al., 2015;McKay et al., 2016). At site GeoB3606-1, diatoms are the main contributors to the opal fraction. The highest diatom accumulation rate and exceptionally high values of opal (up to 45 wt.%), as well as the strongest millennial and sub-millennial 70 scale wt.% fluctuations, occurred from 70 and 30 kyr (Fig. 2). Changes in SST and mixing of the uppermost water column alone are unlikely to fully explain variations in diatom productivity (Romero et al., 2015). The inverse correlation between the relative abundance of the Antarctic diatom Fragilariopsis kerguelensis and the alkenone-based SST variations (Fig. 2) in GeoB3606-1 from 70 to 30 kyr suggests enhanced invasion of DSi-rich SO waters and stronger wind-driven mixing during this interval of high opal burial (Romero et al., 2003;Shukla and Romero, 2018). The warming of the last deglaciation (19-13 75 kyr, Fig. 2) suggests, in contrast, stratification of the uppermost water column, accompanied by a distinctive shift in the diatom assemblage from an upwelling-dominated to a non-upwelling community (Fig. 2), and an increase of calcareous production (Romero et al., 2003). To better understand these past changes, and deconvolve interactions between DSi supply, utilisation in surface waters and burial in sediments, we constructed a record of δ 30 Si from hand-picked specimens of Actinocyclus curvatulus and Coscinodiscus radiatus at site GeoB3606-1 (Fig. 2). Single specimen (or mono-generic) δ 30 Si diatom archives have 80 previously been constructed to assess Si utilisation in particular environments, and reliably record different absolute values and trends compared to measurements from bulk or other siliceous fractions (Doering et al., 2016a, b;Xiong et al., 2015). A. curvatulus is a coastal diatom, mainly thriving in shallow, hemipelagial waters bathing the uppermost slope off Namibia. According to multiyear sediment trap studies off Mauritania, A. curvatulus has a higher contribution to the diatom community before and after the main upwelling season (Romero and Fischer, 2017). C. radiatus is a common component 85 of planktonic assemblages of tropical and subtropical hemi-to pelagial marine areas (Romero and Fischer, 2017). Compared to A. curvatulus, it represents a more "pelagial" signal. Both diatoms are "petri dish-shaped" and can be considered "large": diameter up to 200-220 µm (Hasle et al., 1996). Their valves are significantly larger than that of upwelling diatoms (resting spores of Chaetoceros, up to 25-30 µm length) and other coastal planktonic taxa (25-50 µm). This also means that one A. curvatulus or C. radiatus valve contains possibly 10 to 20 times more opal than the delicate spores of Chaetoceros and consume 90 relatively large amounts of DSi in the build-up of their valves compared to coastal upwelling diatoms. As both species grow away from the major upwelling zone, their isotopic compositions (denoted by δ 30 Si CA for Coscinodiscus/Actinocyclus) will reflect a combination of the initial composition of the ambient water, and their DSi utilisation. Utilisation by both species is unlikely to have a quantitative impact on ambient seawater composition, due to very low overall abundances throughout the record (Romero et al., 2003). Instead, the δ 30 Si CA will reflect utilisation by the dominant upwelling species (resting spores of 95 Chaetoceros spp.).

Laboratory methods
A. curvatulus and C. radiatus valves were hand-picked from washed and sieved sediments (note that low MIS2 data resolution was due to limited sample availability). The picked diatoms were transferred to cleaned Teflon vials and any organic material was removed by drying down in concentrated nitric acid (in-house twice-distilled HNO 3 ) on a hotplate. The diatoms were 100 dissolved in a strong alkaline solution (0.4M sodium hydroxide Analar) at 100 • C overnight. The resulting solution was diluted two-fold in in 18 M Ω.cm Milli-Q water and acidified using 6N hydrochloric acid (in-house twice-distilled HCl) to reach a pH of 2-3. The samples were then purified using cation exchange resin (Georg et al., 2006). Analysis of silicon and magnesium isotopes ( 28 Si, 29 Si, 30 Si, 24 Mg, 25 Mg, and 26 Mg) was carried out in a pilot study by Multi-Collector Inductively-Coupled Plasma Mass Spectrometry (Cassarino et al., 2018;Hendry et al., 2015) at the University of Bristol (Thermo Neptune). Further 105 analyses were carried out using similar methodology at the NERC Isotope Geosciences Laboratory. Each sample was filtered, prior to isotopic analysis, to remove any fine particles, which may have eluted off the cation exchange column during the purification stage (Millex-LG, 0.2 µm, PTFE syringe filter, Millipore). Samples and reference materials were acidified using HCl (to a concentration of 0.05M, using twice quartz distilled acid) and sulphuric acid (to a concentration of 0.003M, using Romil Ultra Purity Acid). This was done following the recommendations of Hughes et al. (2011), the principle being that 110 swamping both samples and reference materials, above and beyond the natural abundance of Cl − and SO 4 2− , will evoke a similar mass bias response in each. Finally, all samples are doped with 300ppb magnesium (Mg, Alfa Aesar SpectraPure).
Spiking with an external element of known isotopic composition allows the data to be monitored and corrected for the effects of instrument induced mass bias (Cardinal et al., 2003). In order to resolve isobaric interferences, principally 14 N 16 O + at mass 30, samples were analysed using the medium mass-resolution capability of a Thermo Scientific Neptune Plus MC-ICP-115 MS (multi collector inductively coupled plasma mass spectrometer), operated in wet-plasma mode. Instrument and sampling details are summarised in Table 1. Using the instrumental parameters outlined, a sensitivity of approximately 4.5V/ppm was obtained. Data were acquired using a dynamic, two sequence acquisition (see Tables 1 and 2 for full operating conditions).
Faraday amplifier gains were measured at the beginning of each analytical session. Data were collected in 1 block of 25 ratios, with a resulting analysis time of approximately 12 minutes per sample (including the sample uptake and stabilisation time of 120 90 seconds). Blanks were measured on the sample make-up acid (0.05M HCl, 0.003M H 2 SO 4 ) using a shortened version of the acquisition procedure above (1 block of 10 ratios). An on-line background correction was made, with the values obtained for the blank acid subtracted from each succeeding sample. Isotope ratios were calculated using following Equ. 1 and 2. Reference standards were run to assess the accuracy and precision of the technique (NIGL diatomite δ 30 Si +1.20 ± 0.16 ‰ (2SD, n = 12); Bristol diatomite +1.30 ± 0.07 ‰ (2SD internal error, n = 1); Bristol LMG08 sponge standard -3.43 ± 0.08 ‰ (2SD internal 125 error, n = 1)), in good agreement with published values (Reynolds et al., 2007;Hendry et al., 2011). Complete replicate measurements were carried out from three different horizons, and reproduced within 0.04 to 0.39 ‰, indicating the natural level of isotopic heterogeneity within a sediment sample. The δ 29 Si and δ 30 Si values of standards and samples showed mass dependent behaviour (slope of 3-isotope plot of 0.50 ± 0.06, consistent with equilibrium or kinetic fractionation; Reynolds et al. (2007)). The procedural blank was below the level of detection.

Modelling
A two-box model "thought experiment" was devised in MATLAB to investigate changes in upwelling and biological productivity between simulated MIS3 and late MIS3/MIS2 conditions, based on De La Rocha and Bickle (2005). The model comprised a surface box (area 100km x 100km, with variable depth), and a deep box (water column height 2500m, area 100km x 100km).
The conditions are described in Table 3.

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A "MIS3" spin up is run for 50 thousand years to reach steady-state, with a deep surface box (100m), high biological production efficiency (i.e. proportion of available DSi used by diatoms) was set at 90%, strong SO input (approximately 1.5 x modern) and export efficiency of 50% (similar to modern). DSi input is set at 70 µM with a δ 30 DSi of +1.2 ‰, reflecting low utilisation in the SO during periods of silicon leakage. Note that although culture studies of Chaetoceros brevis revealed a fractionation factor of -2.09 ‰ during growth (Sutton et al., 2013), the fractionation during Chaetoceros resting spore formation Using a combination of the new isotopic records, together with published values for opal accumulation and diatom assemblage composition, we can recognise four main intervals within our δ 30 Si CA DSi utilisation archive (Fig. 4).

Early MIS 3 (60-40 kyr)
The early MIS3 is characterised by high but variable δ 30 Si CA , ranging between +0.5 and +1.5 ‰. These high δ 30 Si CA values 165 are accompanied by an increase in F. kerguelensis and low SST, indicative of strong utilisation together with a greater input of DSi-rich SO water and intense upwelling. Strong Chaetoceros utilisation would have reduced the concentration of preformed DSi exported away from the shelf and uppermost slope waters towards the A. curvatulus and C. radiatus habitats (hemipelagial and pelagial). Not only was Chaetoceros production high because of the rate of supply of DSi to coastal waters, but also because they were able to use a high proportion of what was available. High opal burial is also found during this time 170 interval in sediments from the Eastern Equatorial Pacific (Kienast et al., 2006), perhaps indicating that this mechanism could have been active in other upwelling zones in the open ocean during MIS3. A similar mechanism may also have been active in the BUS during the Plio-Pleistocene (Etourneau et al., 2012). The time lag (approximately 10 kyr) between the decline in diatom accumulation and δ 30 Si CA could be due to a decrease in the DSi concentration of the supplied water after 49 kyr (i.e. a decline in SO water, indicated by the decrease of F. kerguelensis abundance, Fig. 2). High utilisation of waters with a lower values due to prior Si isotopic enrichment of the water. The resolution of δ 30 Si CA samples also declines into MIS 3 and MIS2, as a result of low diatom abundance, which limits our ability to link temporally the diatom utilisation and opal production throughout this period.

Late MIS 3 to 2 (40-15 kyr)
180 By the late MIS3, the δ 30 Si CA record starts to decline towards lower values, reaching the lowest value of -1.5 ‰ by the end of MIS2 (Fig 2). Weakening upwelling (trends to higher SST) occurred at the same time as the decline towards lower δ 30 Si CA values, which points towards a significant decrease in utilisation as well as low opal production: as less DSi was being supplied from the SO, diatoms generally decline in abundance. MIS2 saw a shift from a siliceous-calcareous productivity system to one dominated by calcareous production began during early MIS2 (Romero et al., 2003).

MIS 1 (15 kyr to present)
MIS1 is characterised by a return to higher δ 30 Si CA values (approximately 0 to +1.5 ‰), similar to MIS4 and early MIS3 ( Fig.   2A), suggesting a return to high DSi utilisation. The dominance of open-ocean, warm-water diatoms, coupled with low diatom and opal accumulation throughout MIS1, has been interpreted to reflect the predominance of DSi-poor water masses at 25 • in the BUS (Romero, 2010). At the same time as the decline in upwelling intensity (from SST and diatom assemblage), and 190 overall low SO water contribution, our new isotope record shows a step change into MIS1 to higher δ 30 Si CA values, similar to MIS3/4 (Fig. 2). This suggests that there was an increase in pelagic diatom utilisation, despite lessened DSi availability and weakened mixing. This is potentially also coupled with a reduction in DSi concentration and accompanying increase in initial seawater Si isotopic composition. A drop in Chaetoceros utilization (of a less enriched DSi water supply, supplied at a lower rate) will result in more DSi being exported offshore from the upwelling zone and a higher DSi availability for the (hemi) 195 pelagic diatoms, which will thrive under more favourable conditions. The high MIS1 δ 30 Si CA values are likely, possibly for the first time in the record, to reflect the utilisation by pelagic diatoms, including both A. curvatulus and C. radiatus.

Quantifying changes to the BUS silica cycle
To quantify the potential sensitivity of the BUS to these different driving mechanisms, we have constructed a mass balance thought-experiment, based on a two-box model. DSi enters the system from rivers into the surface box and via 'upwelling' during DSi uptake by A. curvatulus and C. radiatus is likely to be greater than generally assumed, up to -2 ‰, as observed in some diatom cultures of other species (Sutton et al., 2013).

Response of the BUS to silicic acid leakage 210
In addition to informing on silica cycling within the BUS ecosystem, our downcore silicon isotope archive provides broader insight into the SALH. We interpret the diatom δ 30 Si CA record as indicating that there was strong but variable upwelling of Si-rich waters during MIS4 and MIS3, consistent with leakage of SO waters at this time into the eastern basin of the South Atlantic. If correct, this interpretation would further suggest that this Si-enriched AAIW could have been exported throughout the Atlantic, given the reconstructed shifts in BUS upwelling and utilisation are coincident in time with previous downcore 215 evidence of silica leakage into the western basin during MIS3/4 (Griffiths et al., 2013). This silica leakage did not appear to drive strong diatom production throughout the Atlantic, however, and only influenced diatom production in any significant way in the eastern basin, where the thermocline was sufficiently shallow (Abrantes, 2000;Flores et al., 2000;Abrantes, 2003).
Despite this, the silica leakage could have driven an increase in diatom production relative to other phytoplankton groups in regions of the eastern Atlantic basin, contributing to the drop in atmospheric CO 2 observed in ice cores and the decline into full 220 glacial conditions and so supporting the SALH (Matsumoto and Sarmiento, 2008). However, we would suggest that physical oceanographic changes are likely to be largely responsible for the decline in CO 2 during MIS4, which occurred over a much shorter timescale than both the changes in silica cycling within the BUS and the evidence for silica leakage in the eastern basin (Griffiths et al., 2013;Thornalley et al., 2013). Furthermore, the export of Si-enriched SO waters did not last the duration of the last glaciation, and began to decline into late MIS3/MIS2 (i.e. into the LGM) again arguing against a key driving role for 225 biological carbon uptake in controlling atmospheric CO 2 .

Implications for Paleonutrients and Productivity
The cycling of Si in surface waters, and the relationship with organic matter production, will be a function of both net production (utilisation efficiency and recycling) and export. EBUs are generally characterised by high utilisation and export, with rapid turnover. However, the intensity of upwelling and nutrient composition of upwelled waters through time are likely to be 230 sensitive to climate forcings, and will impact the balance between utilisation and export, the availability of preformed DSi, and the coupling between Si and C cycles (Romero et al., 2003;Romero, 2010;Hendry and Brzezinski, 2014). Using a combination of δ 30 Si, diatom assemblages, opal accumulation, and alkenone-based SST allows investigation of how the diatom community was using supplied DSi in the surface waters during periods of rapid climate variations in low-latitude EBUs. We have shown that this approach can be successfully used in the BUS to produce a continuous record across full glacial conditions     Romero, 2010;Romero et al., 2003Romero et al., , 2015. Vertical shaded bars highlight the time-periods discussed in section 3.1.