Extreme, abrupt Northern Hemisphere climate oscillations during the last glacial cycle (140,000 years ago to present) were modulated by changes in ocean circulation and atmospheric forcing. However, the variability of the Atlantic meridional overturning circulation (AMOC), which has a role in controlling heat transport from low to high latitudes and in ocean CO2 storage, is still poorly constrained beyond the Last Glacial Maximum. Here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle, dominating ocean circulation in the Atlantic, whereas a shallower glacial mode with southern-sourced waters filling the deep western North Atlantic prevailed during glacial maxima. Our results are based on a reconstruction of both the strength and the direction of the AMOC during the last glacial cycle from a highly resolved marine sedimentary record in the deep western North Atlantic. Parallel measurements of two independent chemical water tracers (the isotope ratios of (231)Pa/(230)Th and (143)Nd/(144)Nd), which are not directly affected by changes in the global cycle, reveal consistent responses of the AMOC during the last two glacial terminations. Any significant deviations from this configuration, resulting in slowdowns of the AMOC, were restricted to centennial-scale excursions during catastrophic iceberg discharges of the Heinrich stadials. Severe and multicentennial weakening of North Atlantic Deep Water formation occurred only during Heinrich stadials close to glacial maxima with increased ice coverage, probably as a result of increased fresh-water input. In contrast, the AMOC was relatively insensitive to submillennial meltwater pulses during warmer climate states, and an active AMOC prevailed during Dansgaard-Oeschger interstadials (Greenland warm periods).

Global warming during the Palaeocene-Eocene Thermal Maximum1,2 (PETM, ~56 Ma) is commonly interpreted as being primarily driven by the destabilization of carbon from surficial sedimentary reservoirs such as methane hydrates3. However, the source(s) of carbon remain controversial1,3–5. Resolving this is key to understanding the proximal cause, as well as quantifying the roles of triggers versus feedbacks in driving the event. Here we present new boron isotope data – a proxy for seawater pH – that demonstrate the occurrence of persistently suppressed surface ocean pH across the PETM. Our pH data, alongside a paired carbon isotope record, are assimilated in an Earth system model to reconstruct the unfolding carbon cycle dynamics across the event6,7. We find strong evidence for a much larger (>10,000 PgC) and on average isotopically heavier carbon source than considered previously8,9. This leads us to identify volcanism associated with the North Atlantic Igneous Province, rather than carbon from a surficial reservoir, as the main driver of the PETM10,11. We also find that, although amplifying organic carbon feedbacks with climate likely played only a subordinate role in driving the event, enhanced organic matter burial was important in ultimately sequestering the released carbon and accelerating the recovery of the Earth system12.

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We investigated the Neoproterozoic-early Paleozoic evolution of the Gondwanan margin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation-inductively coupled plasma-mass spectrometry detrital zircon analysis and dating of syn- and posttectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45-0.65 Ga and 0.9-1.3 Ga, with minimal older detritus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evidence of a subduction-related magmatic belt (474-442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that preand postdate this phase of magmatism. These are confi rmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordovician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our fi ndings support models that show these two active margins facing each other during the Ordovician

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.

A high-resolution authigenic Nd isotope record has been extracted from the Fe-Mn oxyhydroxide fraction of drift sediments along the Blake Ridge in the North Atlantic.These sediments facilitate reconstruction of the timing and extent of major hydrographic changes in the western North Atlantic since the Last Glacial Maximum (LGM). This is one of the few locations where sediments were deposited in the major flow path of the Western Boundary Undercurrent (WBUC), which transports North Atlantic Deep Water (NADW) southward at the present day. The hydrodynamic setting, however, also causes problems. Authigenic Nd isotope compositions similar to the typical present-day NADW ε Nd value of -13.5 ± 0.5 were only extracted from sediments located within the main water body of the WBUC coinciding with the highest along slope current velocity below 3200 m water depth. Above this depth the authigenic Nd isotopic composition is more radiogenic than measured in a nearby seawater profile and appears to be influenced by downslope and lateral sediment redistribution. Our data suggest that these radiogenic signals were formed at shallow depths in Florida current waters, compromising the 2The LGM and deglacial sediments below 3400 m water depth bear no evidence of an ambient deep water ε Nd as unradiogenic as -13.5. Although the deep core sites also experienced enhanced degrees of sediment focusing before the Younger Dryas, the ε Nd values of between -11 and -10 are more readily explained in terms of increased presence of Southern Source Waters. If this is the case, the change to Nd isotopic compositions that reflect a modern circulation pattern, including the presence of Lower NADW, only occurred after the Younger Dryas.

Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227(Sites /684, 1228(Sites /680, 1229(Sites /681 and 1230 Sr/ 86 Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the d 13 C values of the dolomite are not in equilibrium with the d 13 C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87 Sr/ 86 Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (<30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate-methane interface. It has been hypothesized that microbial 'hot-spots', such as the sulphate-methane interface, may act as focused sites of dolomite precipitation. Varying d 13 C values from )15& to +15& for the dolomite are consistent with precipitation at a dynamic sulphate-methane interface, where d 13 C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate-methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.

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