(Supplementary Table 1) Mercury isotope values for common and thick-billed murre (Uria aalge, Uria lomvia) eggs from Alaska

Atmospheric deposition of mercury to remote areas has increased threefold since pre-industrial times. Mercury deposition is particularly pronounced in the Arctic. Following deposition to surface oceans and sea ice, mercury can be converted into methylmercury, a biologically accessible form of the toxin, which biomagnifies along the marine food chain. Mass-independent fractionation of mercury isotopes accompanies the photochemical breakdown of methylmercury to less bioavailable forms in surface waters. Here we examine the isotopic composition of mercury in seabird eggs collected from colonies in the North Pacific Ocean, the Bering Sea and the western Arctic Ocean, to determine geographical variations in methylmercury breakdown at northern latitudes. We find evidence for mass-independent fractionation of mercury isotopes. The degree of mass-independent fractionation declines with latitude. Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient. However, mass-independent fractionation was negatively correlated with sea-ice cover. We conclude that sea-ice cover impedes the photochemical breakdown of methylmercury in surface waters, and suggest that further loss of Arctic sea ice this century will accelerate sunlight-induced breakdown of methylmercury in northern surface waters.

(Table S1) Boron isotope and trace element data and reconstructed carbonate system parameters, ODP Holes 122-761B and 154-926A

The middle Miocene Climatic Optimum (17-15 Ma; MCO) is a period of global warmth and relatively high CO2 and is thought to be associated with a significant retreat of the Antarctic Ice Sheet (AIS). We present here a new planktic foraminiferal d11B record from 16.6 to 11.8 Ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to CO2. These new data demonstrate that the evolution of global climate during the middle Miocene (as reflected by changes in the cyrosphere) was well correlated to variations in the concentration of atmospheric CO2. What is more, within our sampling resolution (~1 sample per 300 kyr) there is no evidence of hysteresis in the response of ice volume to CO2 forcing during the middle Miocene, contrary to what is understood about the Antarctic Ice Sheet from ice sheet modelling studies. In agreement with previous data, we show that absolute levels of CO2 during the MCO were relatively modest (350-400 ppm) and levels either side of the MCO are similar or lower than the pre-industrial (200-260 ppm). These new data imply the presence of either a very dynamic AIS at relatively low CO2 during the middle Miocene or the advance and retreat of significant northern hemisphere ice. Recent drilling on the Antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the AIS did occur during the middle Miocene, but the complete loss of the AIS was unlikely. Consequently, it seems that ice volume and climate variations during the middle Miocene probably involved a more dynamic AIS than the modern but also some component of land-based ice in the northern hemisphere.

Alkenone stable carbon isotopes, planktic foraminfera stable carbon isotopes, alkenone unsaturation indices and calculated sea surface temperatures and atmospheric pCO2 for ODP Site 165-999

Temperature reconstructions indicate that the Pliocene was ~3 °C warmer globally than today, and several recent reconstructions of Pliocene atmospheric CO2 indicate that it was above pre-industrial levels and similar to those likely to be seen this century. However, many of these reconstructions have been of relatively low temporal resolution, meaning that these records may have failed to capture variations associated with the 41 Kyr glacial-interglacial cycles thought to operate in the Pliocene. Here we present a new, high temporal resolution alkenone carbon isotope based record of pCO2 spanning 2.8 to 3.3 million years ago from ODP Site 999. Our record is of high enough resolution (~19 Kyrs) to resolve glacial-interglacial changes beyond the intrinsic uncertainty of the proxy method. The record suggests that Pliocene CO2 levels were relatively stable, exhibiting variation less than 55 ppm. We perform sensitivity studies to investigate the possible effect of changing sea surface temperature, which highlights the importance of accurate and precise SST reconstructions for alkenone palaeobarometry, but demonstrate that these uncertainties do not affect our conclusions of relatively stable pCO2 levels during this interval.

d13C and d15N stable isotopes values, total nitrogen and C/N ratios of sediment core GeoB4801-1

Rising stable nitrogen isotope ratios (d15N) in dated sediment records of the German Bight/SE North Sea track river-induced coastal eutrophication over the last 2 centuries. Fully exploiting their potential for reconstructions of pristine conditions and quantitative analysis of historical changes in the nitrogen cycle from these sediment records requires knowledge on processes that alter the isotopic signal in non-living organic matter (OM) of sinking particles and sediments. In this study, we analyze the isotopic composition of particulate nitrogen (PN) in the water column during different seasons, in surface sediments, and in sediment cores to assess diagenetic influences on the isotopic composition of OM. Amino acid (AA) compositions of suspended matter, surface sediments, and dated cores at selected sites of the German Bight serve as indicators for quality and degradation state of PN. The d15N of PN in suspended matter had seasonal variances caused by two main nitrate sources (oceanic and river) and different stages of nitrate availability during phytoplankton assimilation. Elevated d15N values (> 20 per mil) in suspended matter near river mouths and the coast coincide with a coastal water mass receiving nitrate with elevated isotope signal (d15N > 10 per mil) derived from anthropogenic input. Particulate nitrogen at offshore sites fed by oceanic nitrate having a d15N between 5 and 6 per mil had low d15N values (< 2 per mil), indicative of an incipient phytoplankton bloom. Surface sediments along an offshore-onshore transect also reflect the gradient of low d15N of nitrate in offshore sites to high values near river mouths, but the range of values is smaller than between the end members listed above and integrates the annual d15N of detritus. Sediment cores from the coastal sector of the gradient show an increasing d15N trend (increase of 2.5 per mil) over the last 150 years. This is not related to any change in AA composition and thus reflects eutrophication. The d15N signals from before AD 1860 represent a good estimation of pre-industrial isotopic compositions with minimal diagenetic overprinting. Rising d13C in step with rising d15N in these cores is best explained by increasing productivity caused by eutrophication.

Seawater carbonate chemistry in Hog reef, Bermuda reef community, 2010

Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and seawater carbonate chemistry. In this study, contemporaneous in situ datasets of seawater carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising carbon dioxide (CO2) levels and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO32-] and aragonite saturation state omega aragonite, rather than other environmental factors such as light and temperature. These field observations provide sufficient data to hypothesize that there is a seasonal "Carbonate Chemistry Coral Reef Ecosystem Feedback" (CREF hypothesis) between the primary components of the reef ecosystem (i.e., scleractinian hard corals and macroalgae) and seawater carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO32-] and omega aragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO2. In late summer, rates of coral calcification are suppressed by release of CO2 from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO32-] and omega aragonite in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experience seasonal periods of zero net calcification within the next decade at [CO32-] and omega aragonite thresholds of ~184 micro moles kg-1 and 2.65. However, net autotrophy of the reef during winter and spring (as part of the CREF hypothesis) may delay the onset of zero NEC or decalcification going forward by enhancing [CO32-] and omega aragonite. The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by >50% compared to pre-industrial times.

Biological impacts of enhanced alkalinity in Carcinus maenas

Further steps are needed to establish feasible alleviation strategies that are able to reduce the impacts of ocean acidification, whilst ensuring minimal biological side-effects in the process. Whilst there is a growing body of literature on the biological impacts of many other carbon dioxide reduction techniques, seemingly little is known about enhanced alkalinity. For this reason, we investigated the potential physiological impacts of using chemical sequestration as an alleviation strategy. In a controlled experiment, Carcinus maenas were acutely exposed to concentrations of Ca(OH)2 that would be required to reverse the decline in ocean surface pH and return it to pre-industrial levels. Acute exposure significantly affected all individuals' acid-base balance resulting in slight respiratory alkalosis and hyperkalemia, which was strongest in mature females. Although the trigger for both of these responses is currently unclear, this study has shown that alkalinity addition does alter acid-base balance in this comparatively robust crustacean species.

Paleotemperatures, stable isotopes and alkenones of samples from the South Atlantic

Carbon dioxide is one of the most important greenhouse gases which are increasing in atmospheric concentration due to human activities. For using natural CO2 dynamics as a key to understanding the climatic consequences of anthropogenic pCO2 rise, the ocean plays an important role due to its much larger carbon pool compared to the atmosphere. By studying the ratio of stable carbon isotopes in organic matter from marine sediments, it is possible to estimate the partial pressure of CO2 in surface waters during ancient times. The organic compound C37:2 alkenone, whose sole origin is from autotrophic marine algae, was chosen for d13C analysis and its isotopic composition used to reconstruct past PCO2 levels in the surface layer of the eastern Angola Basin for the last 200,000 years. In addition to the variation of ancient concentrations of dissolved CO2 ([CO2(aq)] = ce), the effect of carbon demand which depends on algal growth rate was considered. Here to, carbon isotopic fractionation of C37:2 alkenones (ep) in core-top sediments from the equatorial and the South Atlantic was calibrated against pre-industrial [CO2(aq)] and phosphate concentrations in surface waters. From these data, a variable b = (25 per mil - ep) * ce which reflects intracellular carbon demand was calculated. This variable b correlates with the ambient concentration of seawater phosphate and depends on growth rates. The bulk sediment d15N was used as a proxy parameter for calculating ancient b-values, taking into account that d15N in core-top sediments is correlated to phosphate concentration in modern surface waters. On this basis, the alkenone d13C record of GeoB1016-3 documents a permanent oceanic source for atmospheric carbon dioxide during the last 200,000 years. As a consequence of using d15N derived b-values instead of b = constant, the Angola Basin appears to have been an even stronger CO2 source during glacial periods than at present. Qualitatively similar results were reported by Jasper et al. (1994) for the central Equatorial Pacific. These observations suggest that enhanced productivity of low-latitude upwelling areas during glacial periods is not responsible for the lower CO2 content of the glacial atmosphere.

Holocene precipitation change in different monsoon sub-regions (time-slices and transient data) simulated by different global climate models, with links to model results

The recently proposed global monsoon hypothesis interprets monsoon systems as part of one global-scale atmospheric overturning circulation, implying a connection between the regional monsoon systems and an in-phase behaviour of all northern hemispheric monsoons on annual timescales (Trenberth et al., 2000). Whether this concept can be applied to past climates and variability on longer timescales is still under debate, because the monsoon systems exhibit different regional characteristics such as different seasonality (i.e. onset, peak, and withdrawal). To investigate the interconnection of different monsoon systems during the pre-industrial Holocene, five transient global climate model simulations have been analysed with respect to the rainfall trend and variability in different sub-domains of the Afro-Asian monsoon region. Our analysis suggests that on millennial timescales with varying orbital forcing, the monsoons do not behave as a tightly connected global system. According to the models, the Indian and North African monsoons are coupled, showing similar rainfall trend and moderate correlation in rainfall variability in all models. The East Asian monsoon changes independently during the Holocene. The dissimilarities in the seasonality of the monsoon sub-systems lead to a stronger response of the North African and Indian monsoon systems to the Holocene insolation forcing than of the East Asian monsoon and affect the seasonal distribution of Holocene rainfall variations. Within the Indian and North African monsoon domain, precipitation solely changes during the summer months, showing a decreasing Holocene precipitation trend. In the East Asian monsoon region, the precipitation signal is determined by an increasing precipitation trend during spring and a decreasing precipitation change during summer, partly balancing each other. A synthesis of reconstructions and the model results do not reveal an impact of the different seasonality on the timing of the Holocene rainfall optimum in the different sub-monsoon systems. They rather indicate locally inhomogeneous rainfall changes and show, that single palaeo-records should not be used to characterise the rainfall change and monsoon evolution for entire monsoon sub-systems.