Does sea level influence mid-ocean ridge magmatism on Milankovitch timescales?

Magma production at mid-ocean ridges is driven by seafloor spreading and decompression melting of the upper mantle. In the special case of Iceland, mantle melting may have been amplified by ice sheet retreat during the last deglaciation, yielding anomalously high rates of subaerial volcanism. For the remainder of the global mid-ocean ridge system, the ocean may play an analogous role, with lowering of sea level during glacial maxima producing greater magma flux to ridge crests. Here we show that the mantle decompression rate associated with changes in sea level is a substantial fraction of that from plate spreading. Modeled peaks in magma flux occur after sea level drops rapidly, including the Marine Isotope Stage (MIS) 5/4 and 3/2 transitions. The minimum in simulated flux occurs during the mid-Holocene, due to the rapid sea level rise at the MIS 2/1 boundary. The model results are highly sensitive to melt migration rate; rates of ~1 m/yr produce small signals, while those >5 m/yr yield substantial anomalies. In the latter case, sea level-driven magma flux varies by 15–100% relative to the long-term average, with the largest effect occurring at slow-spreading ridges. We suggest that sedimentary time series of hydrothermal particle flux, oceanic Os isotopic ratio, and oceanic radiocarbon may serve as proxies for magma-flux variations at mid-ocean ridges. Although well-dated records are rare, preliminary data from the Pacific and Atlantic suggest hydrothermal metal flux was elevated during MIS 2 and 4, broadly consistent with our modeling results

Hydrothermal sediments as a potential record of seawater Nd isotope compositions: The Rainbow vent site (36°14?N, Mid-Atlantic Ridge)

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ~2 and 5 km from the Rainbow hydrothermal vent site on the Mid-Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy-anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe-Mn oxy-hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the Nd value of present-day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater Nd values. Nd isotope data from these cores span the period of 4–14 ka (14C ages) and yield Nd values for North East Atlantic Deep Water (NEADW) that are higher (?9.3 to ?11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (?12.4 ± 0.9). This observation suggests that either the Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid-Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher-resolution record of deep water Nd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments

Global environmental effects of large volcanic eruptions on ocean chemistry: Evidence from “hydrothermal” sediments (ODP Leg 185, Site 1149B)

A Sr-Nd-Pb isotope investigation has been carried out on sediments overlying the Cretaceous oceanic crust at ODP Leg 185 Site 1149B. The sediments (biogenic carbonate and siliceous, clays) contain two horizons with high “excess” concentrations of Fe, Zn, Cu, and Pb. These horizons are both characterized by rare Earth element patterns that are concave-upwards and exhibit both negative Ce and positive Eu anomalies; comparable to those from modern hydrothermal plume particles and sediments. However, Pb isotope signatures differ significantly between the two metal-rich horizons. Metal-rich sediments in the lower part of Unit IV have Pb isotopes that lie on a mixing line between seawater and local oceanic crust end-members, which is consistent with a hydrothermal fall-out originating from a local oceanic spreading centre. In contrast, sediments from the upper part of Unit IV have much more radiogenic Pb isotope ratios, that cannot be related readily to local end-members (off-axis hydrothermal vent, clays, oceanic crust or large igneous provinces (LIP)). Their age-corrected Pb isotope compositions do, however, overlap with basalts from the Parana-Etendeka LIP. Evidence for related environmental effects include a drastic biotic change, higher oceanic ? 13C values and more radiogenic seawater 87Sr/86Sr. We estimate that the Parana-Etendeka LIP released ?1 × 1019 g of carbon into the atmosphere as CO2, corresponding to Pb and S degassing fluxes of ?1 × 109 g a?1 and ?15 × 1012 g a?1, respectively. The study demonstrates that Pb isotopes when combined with other geochemical parameters are ideal tools for detecting and tracing LIP volcanism in the marine geological records. <br/