Marine gateways play a critical role in the exchange of water, heat, salt and nutrients between oceans and seas. As a result, changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate, as well as having a profound impact on local environmental conditions. Mediterranean-Atlantic marine corridors that pre-date the modern Gibraltar Strait, closed during the Late Miocene and are now exposed on land in northern Morocco and southern Spain. The restriction and closure of these Miocene connections resulted in extreme salinity fluctuations in the Mediterranean, leading to the precipitation of thick evaporites. This event is known as the Messinian Salinity Crisis (MSC). The evolution and closure of the Mediterranean-Atlantic gateways are a critical control on the MSC, but at present the location, geometry and age of these gateways is still highly controversial, as is the impact of changing Mediterranean outflow on Northern Hemisphere circulation. Here, we present a comprehensive overview of the evolution of the Late Miocene gateways and the nature of Mediterranean-Atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins. We also consider the possible impact of evolving exchange on both the Mediterranean and global climate and highlight the main enduring challenges for reconstructing past Mediterranean-Atlantic exchange.

In response to rising CO2 concentrations and increasing global sea surface temperatures, oxygen minimum zones (OMZ), or “dead zones”, are expected to expand. OMZs are fueled by high primary productivity, resulting in enhanced biological oxygen demand at depth, subsequent oxygen depletion, and attenuation of remineralization. This results in the deposition of organic carbon‐rich sediments. Carbon drawdown is estimated by biogeochemical models; however, a major process is ignored: carbon fixation in the mid‐ and lower water column. Here, we show that chemoautotrophic carbon fixation is important in the Arabian Sea OMZ; and manifests in a 13C‐depleted signature of sedimentary organic carbon. We determined the δ13C values of Corg deposited in close spatial proximity but over a steep bottom‐water oxygen gradient, and the δ13C composition of biomarkers of chemoautotrophic bacteria capable of anaerobic ammonia oxidation (anammox). Isotope mixing models show that detritus from anammox bacteria or other chemoautotrophs likely forms a substantial part of the organic matter deposited within the Arabian Sea OMZ (~17%), implying that the contribution of chemoautotrophs to settling organic matter is exported to the sediment. This has implications for the evaluation of past, and future, OMZs: biogeochemical models that operate on the assumption that all sinking organic matter is photosynthetically derived, without new addition of carbon, could significantly underestimate the extent of remineralization. Oxygen demand in oxygen minimum zones could thus be higher than projections suggest, leading to a more intense expansion of OMZs than expected.

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Rationale: The hydrogen isotopic composition of lipids (δ 2 Hlipid) is widely used in food science and as a proxy for past hydrological conditions. Determining the δ 2 H values of large, well-preserved triacylglycerides and other uniquely microbial lipids, such as glycerol dialkyl glycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been This is a non-peer reviewed manuscript submitted to Rapid Communications in Mass Spectrometry. 2 possible due to their size which prohibits analysis by traditional gas chromatography pyrolysis isotope ratio mass spectrometry (GC-P-IRMS). Methods: We determined the δ 2 H values of large, polar molecules and applied high temperature gas chromatography (GC) methods on a modified GC-P-IRMS system. The methods were validated using authentic standards of large, functionalised molecules (triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2 H values determined by elemental analyser pyrolysis isotope ratio mass spectrometry (EA-P-IRMS); and subsequently applied to the analysis of GDGTs in a sample from a methane seep and a Welsh peat. Results: δ 2 H values of TAGs agreed within error between different between GC-P-IRMS and EA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipid GDGTs with up to three cyclisations could be analysed: δ 2 H values were not significantly different between methods with standard deviations of 5 to 6 ‰. When environmental samples were analysed, δ 2 H values of isoGDGTs were 50 ‰ more negative than those of terrestrial brGDGTs. Conclusions: Our results indicate that the high temperature GC-P-IRMS (HTGC-P-IRMS) method developed here is appropriate to determine the δ 2 H values of TAGs, GDGT lipids with up to two cyclisations, and potentially other high molecular weight compounds. The methodology will widen the current analytical window for biomarker and alimentary light stable isotope analyses. Moreover, our initial measurements suggest that bacterial and archaeal GDGT δ 2 H values can record environmental and ecological conditions.

<div>Rationale: The hydrogen isotopic composition of lipids (δ 2Hlipid) is widely used in food</div><div>science and as a proxy for past hydrological conditions. Determining the δ 2H values of large,</div><div>well-preserved triacylglycerides and other uniquely microbial lipids, such as glycerol dialkyl</div><div>glycerol tetraether (GDGT) lipids, is thus of widespread interest but has so far not been </div><div>possible due to their size which prohibits analysis by traditional gas chromatography</div><div>pyrolysis isotope ratio mass spectrometry (GC-P-IRMS).</div><div>Methods: We determined the δ 2H values of large, polar molecules and applied high</div><div>temperature gas chromatography (GC) methods on a modified GC-P-IRMS system. The</div><div>methods were validated using authentic standards of large, functionalised molecules</div><div>(triacylglycerides, TAG), purified reference standards of GDGTs, and compared to δ 2H</div><div>values determined by elemental analyser pyrolysis isotope ratio mass spectrometry (EA-PIRMS); and subsequently applied to the analysis of GDGTs in a sample from a methane</div><div>seep and a Welsh peat.</div><div>Results: δ 2H values of TAGs agreed within error between different between GC-P-IRMS and</div><div>EA-P-IRMS, with GC-P-IRMS showing 3-5 ‰ precision for 10 ng H injected. Archaeal lipid</div><div>GDGTs with up to three cyclisations could be analysed: δ 2H values were not significantly</div><div>different between methods with standard deviations of 5 to 6 ‰. When environmental</div><div>samples were analysed, δ 38 2H values of isoGDGTs were 50 ‰ more negative than those of</div><div>terrestrial brGDGTs.</div><div>Conclusions: Our results indicate that the high temperature GC-P-IRMS (HTGC-P-IRMS)</div><div>method developed here is appropriate to determine the δ 2H values of TAGs, GDGT lipids</div><div>with up to two cyclisations, and potentially other high molecular weight compounds. The</div><div>methodology will widen the current analytical window for biomarker and alimentary light</div><div>stable isotope analyses. Moreover, our initial measurements suggest that bacterial and</div><div>archaeal GDGT δ 2H values can record environmental and ecological conditions</div>