The cause of atmospheric CO 2 change during the recent ice ages remains a first order question in climate science. Most mechanisms have invoked carbon exchange with the deep ocean, due to its large size and relatively rapid exchange time with the atmosphere 1 . The Southern Ocean is thought to play a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region 2 . However reconstructing changes in deep Southern Ocean carbon storage is challenging, so few direct tests of this hypothesis exist. Here we present new deep-sea coral boron isotope data that track the pH -and thus CO 2 chemistry -of the deep Southern Ocean over the last 40,000 years. At sites closest to the Antarctic continental margin, and most influenced by the deep Southern waters that form the ocean's lower overturning cell, we find a close relationship between ocean pH and atmospheric CO 2 : during intervals of low CO 2 ocean pH is low, reflecting enhanced ocean carbon storage; during intervals of rising CO 2 ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial to centennial-scale) pH decreases during abrupt CO 2 rise, reflecting the rapid transfer of carbon from the deep to the upper

Understanding how rising seawater pCO 2 and temperatures impact coral aragonite accretion is essential for predicting the future of reef ecosystems. Here, we report 2 long-term (10-11 month) studies assessing the effects of temperature (25 and 28°C) and both high and low seawater pCO 2 (180-750 latm) on the calcification, photosynthesis and respiration of individual massive Porites spp. genotypes. Calcification rates were highly variable between genotypes, but high seawater pCO 2 reduced calcification significantly in 4 of 7 genotypes cultured at 25°C but in only 1 of 4 genotypes cultured at 28°C. Increasing seawater temperature enhanced calcification in almost all corals, but the magnitude of this effect was seawater pCO 2 dependent. The 3°C temperature increase enhanced calcification rate on average by 3% at 180 latm, by 35% at 260 latm and by [ 300% at 750 latm. The rate increase at high seawater pCO 2 exceeds that observed in inorganic aragonites. Responses of gross/net photosynthesis and respiration to temperature and seawater pCO 2 varied between genotypes, but rates of all these processes were reduced at the higher seawater temperature. Increases in seawater temperature, below the thermal stress threshold, may mitigate against ocean acidification in this coral genus, but this moderation is not mediated by an increase in net photosynthesis. The response of coral calcification to temperature cannot be explained by symbiont productivity or by thermodynamic and kinetic influences on aragonite formation.

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Coral skeletal Sr/Ca is a palaeothermometer commonly used to produce high resolution seasonal sea surface temperature (SST) records and to investigate the amplitude and frequency of ENSO and interdecadal climate events. The proxy relationship is typically calibrated by matching seasonal SST and skeletal Sr/Ca maxima and minima in modern corals. Applying these calibrations to fossil corals assumes that the temperature sensitivity of skeletal Sr/Ca is conserved, despite substantial changes in seawater carbonate chemistry between the modern and glacial ocean. We present Sr/Ca analyses of 3 genotypes of massive Porites spp. corals (the genus most commonly used for palaeoclimate reconstruction), cultured under seawater pCO2 reflecting modern, future (year 2100) and last glacial maximum (LGM) conditions. Skeletal Sr/Ca is indistinguishable between duplicate colonies of the same genotype cultured under the same conditions, but varies significantly in response to seawater pCO2 in two genotypes of Porites lutea, whilst Porites murrayensis is unaffected. Within P. lutea, the response is not systematic: skeletal Sr/Ca increases significantly (by 2–4%) at high seawater pCO2 relative to modern in both genotypes, and also increases significantly (by 4%) at low seawater pCO2 in one genotype. This magnitude of variation equates to errors in reconstructed SST of up to −5 °C.

1The East Scotia subduction zone, located in the Atlantic sector of the Southern Ocean, 2 hosts a number of hydrothermal sites in both back-arc and island-arc settings. High 3 temperature (> 348 °C) 'black smoker' vents have been sampled at three locations along 4 segments E2 and E9 of the East Scotia back-arc spreading ridge, as well as 'white smoker' (< 5 212 °C) and diffuse (< 28 °C) hydrothermal fluids from within the caldera of the Kemp 6 submarine volcano. The composition of the endmember fluids (Mg = 0 mmol/kg) is 7 markedly different, with pH ranging from <1 to 3.4, [Cl -] from ~90 to 536 mM, [H 2 S] from 8 6.7 to ~200 mM and [F -] from 35 to ~1000 μM. All of the vent sites are basalt-to basaltic 9 andesite-hosted, providing an ideal opportunity for investigating the geochemical controls on 10 rare earth element (REE) behaviour. Endmember hydrothermal fluids from E2 and E9 have 11 total REE concentrations ranging from 7. 2). 16We demonstrate that the REE geochemistry of fluids from the East Scotia back-arc 17 spreading ridge is variably influenced by ion exchange with host minerals, phase separation, 18 competitive complexation with ligands, and anhydrite deposition, whereas fluids from the 19 Kemp submarine volcano are also affected by the injection of magmatic volatiles which 20 enhances the solubility of all the REEs. We also show that the REE patterns of anhydrite 21 deposits from Kemp differ from those of the present-day fluids, potentially providing critical 22 information about the nature of hydrothermal activity in the past, where access to 23 hydrothermal fluids is precluded. 24 25 3 INTRODUCTION 26The chemical properties of the rare earth elements (REEs) are fundamentally similar, 27 and differences in their behaviour in natural materials and fluids can usually be attributed to 28 atomic radii controls on their speciation and mobility (Elderfield et al., 1988). In 29 hydrothermal environments, the distribution of the REEs provides important information 30 about fluid evolution during subsurface circulation (Elderfield et al., 1988;Haas et al., 1995), 31 sources of fluid constituents and the extent of seawater mixing (Van Dover, 2000; Embley et 32 al., 2007;Craddock et al., 2010), conditions of mineral deposition and venting history 33 (Craddock and Bach, 2010) and the transport and fate of plume particulate material (German 34 et al., 1990;Bau and Dulski, 1999;Sherrell et al., 1999). 35Most hydrothermal fluids have remarkably uniform chondrite-normalised REE 36 (REE CN ) distribution patterns, with enrichment in the light-REEs relative to the heavy-REEs 37 and a positive europium anomaly (Michard and Albarède, 1986). This pattern has been 38 observed in fluids from basalt-hosted hydrothermal systems on the Mid-Atlantic Ridge 39 (MAR) (Mitra et al., 1994;James et al., 1995) and the East Pacific Rise (EPR) (Michard et 40 al., 1983;Michard and Albarède, 1986), but also in fluids venting through basaltic andesite 41 and andesite substrates in the Lau Basin , heavily-sedimented...