Enhanced East Pacific Rise hydrothermal activity
Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions.
Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations.
We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.
Sea level–driven pressure variations due to the growth and decay of ice sheets likely modulate melt production in the upper mantle on Milankovitch time scales (1,2). Model simulations suggest that the magnitude of the resulting signal at mid-ocean ridges depends on the plate spreading rate, the melt extraction velocity, and the thermal properties of the lithosphere (1,3). Because of the slow rate of melt migration in the upper mantle, the magmatic signal at ridges probably lags changes in sea level by thousands of years (1).
Surveys of ridge bathymetry reveal Milankovitch-scale frequencies in abyssal-hill spacing, consistent with the sea-level hypothesis (3,4). Bathymetry records are subject to geological damping effects and substantial age uncertainties, however, and they therefore require validation with other proxies. Because hydrothermal activity along ridge sections is ultimately driven by magmatic heat, sedimentary records of hydrothermal output can be used to assess the sea-level hypothesis and determine the timing of magmatic anomalies relative to key Pleistocene climate transitions.
The southern East Pacific Rise (SEPR) has the fastest spreading rate and the highest magmatic budget of any ridge in the global mid-ocean ridge system (5). Due to its elevated magmatism, the SEPR has over 50 known active vent sites from 5°S to 37°S (6), consistent with the global trend in plume incidence versus magmatic budget for ridges spanning a range of spreading rates (5,7).
Intense hydrothermal venting and topographically steered flow of plumes along the SEPR create a spatially integrated pattern of metalliferous sediments near the ridge crest (8–10).
Compared with slower ridges, SEPR sediments have anomalously high metal concentrations (8,11), suggesting that magmatism is the primary factor governing hydrothermal input to these sedimentary archives on geologic time scales. Hydrothermal plume particles are highly enriched in elements that are derived directly from vents and scavenged from seawater.
Variations in the flux of these elements to ridge-flank sediments should therefore reflect long-term changes in hydrothermal activity.
We used a multiproxy geochemical strategy to reconstruct SEPR hydrothermal activity during the last glacial cycle. We analyzed a total of seven ridge-crest cores from 6°S and 11°S, where the half -spreading rate averages 75 mm/year (Fig. 1). Together with two published records from near the East Pacific Rise (EPR)–Dietz volcanic-ridge triple junction at 1°N (12), the locations span a range of spreading rates, sedimentary environments, and surface-ocean productivity regimes.
To control for spatial heterogeneity in plume incidence versus magmatic budget (7), the sampling locations span three separate EPR segments. At each segment, we analyzed cores from both sides of the ridge axis to address potential biases due to horizontal sediment focusing, bioturbation, and spatial variability in hydro thermal-plume direction.
Radiocarbon and oxygen isotopic analyses of planktonic foraminifera provided age control for each core (13). Major and trace element concentrations were determined using x-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS).
The fluxes of hydrothermal components were estimated using both mass accumulation rates and the 3 He normalization method (13). Given that plume particles primarily consist of Fe oxyhydroxides and Mn oxides (14), we used sedimentary Fe and Mn to track hydrothermal inputs. To cross-check the Fe and Mn results, we also measured arsenic, which is scavenged from seawater by Fe oxyhydroxides and varies coherently with Fe in hydrothermal-plume particles (15)andSEPRsediments(16).
Oxygen stable isotope records from 1°N, 6°S, and 11°S outline marine isotope stages 1, 2, and 3, indicating that there has been minimal stratigraphic disturbance of the cores due to sediment winnowing or downslope transport (Fig. 2). The flux of Fe in all nine records peaks between 10 and 20 thousand years before the present (ky B.P.). Manganese fluxes to EPR sediments follow a similar pattern, with maximum values centered at ~15 ky B.P. Arsenic fluxes at 6°S and 11°S reach a maximum between 10 and 20 ky B.P., supporting the Fe and Mn results.
Offsets between time series are generally 5 ky or less (Fig. 2), similar to the age uncertainty associated with the mass accumulation rate method (13). Results from the 3 He normalization technique, which yields fluxes that are insensitive to age-model uncertainty, show that positive shifts in metal fluxes at 11°S and 6°S occurred within 2 ky of one another (fig. S1). Thus, the overall pattern for the past 50 ky is one of coherent variations in hydrothermal sedimentation along 1300 km of the EPR, with maximum metal inputs coinciding with the last deglaciation (Termination I).
Two cores at 11°S span the penultimate de-glaciation (Termination II), including core Y71-07-53 on the western flank of the SEPR and core Y71-07-47 on the eastern flank (Fig. 3). Metal fluxes are higher in the western-flank core, consistent with the east-west contrast in the shorter records (Fig. 2) and the spatial pattern in metal concentrations of late Holocene sediments (8). In the western-flank core, the flux of each metal increases markedly at ~140 ky B.P., reaches a maximum by 130 ky B.P., and then returns to backgroundlevelsby120kyB.P.(Fig.3).
A similar pattern occurs in the eastern-flankcore. The contemporaneous signal at these locations indicates that hydrothermal inputs on each side of the ridge crest varied in phase. The records also show that the maximum flux of hydrothermal metals coincided with Termination II, similar to the pattern for Termination I. Diagenetic overprinting, horizontal sediment focusing, and dilution with nonhydrothermal components can complicate the interpretation of hydrothermal proxies.
Diagenetic remobilization should influence Fe oxyhydroxides and MnO 2 differently, given the large offset in their redox potentials (17), yet we observed coherent down- core variations in Fe, Mn, and As, regardless of location. Furthermore, down-core Fe/Mn ratios generally fall within the expected range for hydrothermal input (8). Anomalously high Fe/Mn ratios at 1°N are probably due to suboxic diagenesis and Mn remobilization(13). At the 1°N locations, near-zero Mn levels before 20 ky BP are likely driven by MnO 2 reduction and upward migration of dissolved Mn 2+.
Nevertheless, the overall coherent pattern in Fe records from the high-productivity equatorial Pacific (1°N) to the northern edge of the subtropical gyre (11°S) indicates that the organic carbon flux to the sediments is not a irst-order control on down-core metal variability. Sediment focusing is an equally unlikely explanation, given the similar pattern in multiple cores from a range of sedimentary environments.
Focusing factors estimated using 3 He also show no evidence for anomalous horizontal sediment transport during Termination I (fig. S3). Lastly, the 3 He-based metal fluxes are consistent with the mass accumulation rate results, indicating that carbonate dilution was not a primary driver of the down-core signal. Taken together, these lines of evidence indicate that the metal fluxes primarily reflect the input of hydrothermal-plume particles to ridge-crest sediments.
The temporal variability in metal fluxes is similar to that in seafloor bathymetry at 17°S on the SEPR, implying that both have a common driver (Fig. 4). A lowering of sea level due to ice-sheet expansion would promote decompression melting in the upper mantle. The resulting increase in melt delivery to the ridge crest should result in shoaling of the bathymetry and greater hydrothermal activity (1,3).
Ice-sheet retreat and rising sea level would have the opposite effect. Shallower bathymetry on the SEPR generally corresponds to elevated hydrothermal fluxes, consistent with the expected pattern (Fig. 4). The bathymetry record lags the hydrothermal proxies by ~10 ky, however (fig. S4). The offset is most likely due to age uncertainty in the bathythmetry timeseries, where the age model is based on a half-spreading rate that optimizes the match between the bathymetry and atmospheric CO2 records (4).
More generally, age constraints for late Pleistocene oceanic crust are limited to two control points, an assumed zero age at the ridge crest and the Brunhes-Matuyama boundary at 780 ky B.P. Even if reliable absolute ages were available for individual abyssal hills, it is unlikely that their bathymetry would reflect only the melt delivery that occurred when that oceanic crust was at the ridge crest, because of the confounding influences of lower crustal accretion, surface lava flows, and vertical and horizontal offsets of crustal blocks by faulting (3,4,18,19).
Although bathymetric time series are useful for identifying Milankovitch frequencies, the absolute timing of events is poorly constrained by these records. Hydrothermal proxies, on the other hand, can be accurately dated using radiocarbon and oxygen isotope stratigraphy. As a result, we are able to infer that intervals of intense hydrothermal activity on the EPR occurred during the last two glacial terminations
The coincidence in timing between hydrothermal maxima and glacial terminations implies that there may be a direct causal relationship between sea-level rise and hydrothermal activity. Our understanding of the physical mechanisms of decompression melting and melt migration to the ridge axis suggests a more complex relationship, however. Proxies of magmatic flux should lag sea-level changes by thousands of years, because of the slow rate of melt migration from the magma source region to the ridge axis (1).
During the Last Glacial Maximum, the maximum rate of sea-level decrease (and hence of pressure release in the melting regime) occurred between 30 and 25 ky B.P. (20), or 15 ± 5 ky before the inferred maximum in EPR hydrothermal activity (Fig. 2). We observed a similar lag between the maximum rate of sea-level rise at ~15 ky B.P. (20)and the late Holocene minimum in metal flux.
Assuming an average melt origin depth of 50 km (21), the implied melt extraction velocities range from 2.5 to 5 m/year, which is consistent with the rate of >1 m/year implied by U/Th is equilibrium in zero-age mid-ocean ridge basalts (22) but much lower than the estimates of >50 m/year based on the time lag between deglaciation and volcanism in Iceland (23).
Our estimate is independent of previous methods and provides a range of constraints for refining models of melt extraction at fast spreading centers. Our results support the hypothesis that enhanced ridge magmatism, hydrothermal output, and perhaps mantle CO2 flux act as a negative feedback on ice-sheet size (1,4). Although the modern carbon output from ridges is small (0.02to 0.2 Pg C/year) (24), the flux probably increased as a result of sea-level modulation.
Carbon sources at off-axis locations, back arc basins, and island arcs may also amplify the mid-ocean ridge signal (2). The long melt-migration times for carbon-rich melts may lead to considerable differences in timing between hydrothermal and carbon-flux variations, however (25).
Another mechanism whereby magmatic variations may influence climate is the hydrothermal heat flux itself. Enhanced geothermal heat flux should warm and destabilize the deep ocean (26), with excess heat emerging along isopycnals into the surface Southern Ocean (26,27).
Temperatures in the deep eastern tropical Pacific and Antarctica peaked during each of the last two glacial terminations (28), consistent with the timing of enhanced EPR hydrothermal activity.
The EPR results establish the timing of hydrothermal anomalies, an essential prerequisite for deter- mining whether ridge magmatism can act as a negative feedback on ice-sheet size. The data presented here demonstrate that EPR hydrothermal output increased after the two largest glacial maxima of the past 200,000 years, implicating mid-ocean ridge magmatism in glacial terminations.
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Jerry Krause
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Hi PSI Readers,
This is just a note to inform readers not familiar, with what ‘science.org’, actually is. For given the a fact that in these times there are so many, science this or that, that I did not recognize what ‘science.org’ actually was that I had a long time ago been a member of it who paid yearly fees to be a member. For the periodical SCIENCE is the USA equivalent of the UK periodical NATURE which may be more familiar to even USA general readers of topics SCIENTIFIC.
And if one goes to read more at science.org you will find you cannot read the actual scientific article which has been publisher in the periodical SCIENCE. Hence, what we PSI readers are reading is an advertisement to invited readers to become a member of AAAS (American Association for the Advancement of Science)
Now I expect that some PSI Readers aware that PSI, as a source of SCIENTIFIC INFORMATION, is not highly respected by most members of AAAS. So, if I have to pay a subscription fee to read the actual SCIENCE article, I asked myself: how did PSI get this long summary, written for a general reader?
Only the PSI editors can answer this and I hope that either John or Andy (coeditors now) might do this for me. So I do not need to assume anything.
Have a good day, Jerry
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Jerry Krause
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Hi PSI Readers.
This article documents how academic scientists and journalists write.
The first four paragraphs are word for word from the abstract of SCIENCE’s scientific article.
“Mid-ocean ridge magmatism is driven by seafloor spreading and decompression melting of the upper mantle. Melt production is apparently modulated by glacial-interglacial changes in sea level, raising the possibility that magmatic flux acts as a negative feedback on ice-sheet size. The timing of melt variability is poorly constrained, however, precluding a clear link between ridge magmatism and Pleistocene climate transitions. Here we present well-dated sedimentary records from the East Pacific Rise that show evidence of enhanced hydrothermal activity during the last two glacial terminations. We suggest that glacial maxima and lowering of sea level caused anomalous melting in the upper mantle and that the subsequent magmatic anomalies promoted deglaciation through the release of mantle heat and carbon at mid-ocean ridges.”
As if dividing the abstract into four short paragraphs would make the abstract easier to understand. I have previously written that Louis Elzevir wrote, in his preface to readers of Two New Sciences (1638) by Galileo, had written, as translated by Crew and de Salvio (1914): “Intuitive knowledge keeps paces with accurate definition.” I question what percentage of SCIENCE’s general reader know what a definition of magmatism is? I certainly did not. Nor do I know what accurate definitions of “decompression melting”, “hydrothermal activity”, or “glacial terminations” are. So as, a general reader, whom claims to be a physical scientist, I am lost before completing the first line.
However, if one goes to science.org one finds the following written before the abstract to entice a reader to go to the actual scientific article.
“Sediments on the ocean floor may provide clues about the interplay between ice ages and mid-ocean ridge magma production. Lund et al. present well-dated and detailed sediment records from hydrothermal activity along the East Pacific Rise. The sediments show changes in metal fluxes that are tied to the past two glaciations. Ice age changes in sea level alter magma production, which is manifested by changes in hydrothermal systems. The apparent increase in hydrothermal activity at the East Pacific Rise around the past two glacial terminations suggests some role in moderating the size of ice sheets. Science, this issue p. 478″. Better for me to understand except I have no clue what “metal fluxes” might be relative to sediments or magna.
Maybe my writing is just as poorly defined, but I certainly hope not. Any readers must make this judgment for themselves.
Have a good day, Jerry
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Robert Beatty
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The gravitational energy that goes into keeping our planet in orbit around the Sun, transforms into extra mass on Earth. A simple application of E=Mc^2
The periodic result is sea floor spreading and subduction under the continental shelves, with sea floor spreading being more dominant.
What a wonderful world we live on.
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