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During the last deglaciation (ca. 19-6.5 ka), increased freshwater discharge to the North Atlantic likely caused reductions in Atlantic meridional overturning circulation (AMOC) strength. However, the locations and rates of freshwater discharge are not well constrained, particularly those during the centennial-scale climate oscillations of the Bolling-Allerod warm periods (ca. 14.6-12.9 ka). Here we reconstruct the salinity-dependent [delta][sup.18][O.sub.sw] (sw, seawater) adjacent to the eastern outlets of North America, using paired Mg/Ca and [delta][sup.18]O records on planktonic foraminifera, to investigate whether increased discharge to the North Atlantic caused reductions in AMOC during the Bolling-Allerod and earlier periods of deglaciation. In general, [delta][sup.18][O.sub.sw] decreased and inferred freshwater discharge increased during periods of reduced AMOC. During the Bolling-Allerod, [delta][sup.18][O.sub.sw] decreases coincided with three reductions in AMOC strength ca. 14.1, 13.8, and 13.3 ka. Freshwater discharge modeling suggests that discharge increases of 0.03-0.05 Sverdrups ([10.sup.6] [m.sup.3] [s.sup.-l]) would explain these [delta][sup.18][O.sub.sw] decreases, which were sufficient to force reductions in AMOC strength. Concurrent changes in North Atlantic temperature, and subtropical and tropical atmospheric circulation and precipitation imply that small variations in the North Atlantic hydrologic system may have significant impacts on Northern Hemisphere climate. doi: 10.1130/G30389.1

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reduction as dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE (∑REE) concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the re-mineralization of particulate organic matter. The highest pore water ∑REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale-normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shale-normalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in ∑REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions. [ABSTRACT FROM AUTHOR]

To constrain the fluxes of methane (CH4) in the water colunm above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (-63 to -66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L-1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thennogenic methane sources were detected (-56 to -28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (-25 to -6‰ PDB). Methane oxidation rates of 0.09 to 4.1‰ per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of "downcurrent" plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h-1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × l04 mol h-1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally. [ABSTRACT FROM AUTHOR]