Recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the Pleistocene influence mantle-melting and volcanic eruptions at mid-ocean ridges, with models predicting variations in oceanic crustal thickness and seafloor bathymetry linked to sea level change. Analyses of seafloor bathymetry have found evidence of significant spectral energy at frequencies consistent with Milanković cycles of 1/23, 1/41, and 1/80-1/120 ka−1. However, other studies emphasize the need for crustal thickness observations to test the “sea level hypothesis”.
Here we investigate the hypothesis of climate driven periodicity in mid-ocean ridge magmatism through analysis of a unique bathymetry and crustal thickness dataset derived from a 3D multi-channel seismic investigation of the East Pacific Rise from 9°42' to 57′N. Crustal thickness data spans the last ∼235 ka in age and reveals three axis-parallel zones of 200-800 m thicker crust. The amplitude and spacing of these thick crust ridges, which are most prominent on the east flank of the ridge, are consistent with predictions of sea level modulated mantle melting. Similarly spaced ridges are apparent in the longer duration (470 ka) seafloor bathymetry data. Spectral analysis of these datasets shows peaks centered near 1/80 ka−1 and locally near 1/41 ka−1 on the east flank in both bathymetry and crustal thickness. West flank spectral results show intermittent peaks near 1/100 ka−1 and 1/41 ka−1 in crustal thickness and no coherent peak frequencies in bathymetry data. We attribute differences between the east and west flank to the impacts of spatially variable asymmetric spreading and small changes in the locus of accretion. Observed half spreading rates are dominantly faster to the east with small ridge jumps transferring crust from the west flank. Lagged cross-correlations between sea level and crustal thickness indicate a maximum when the latter is lagged by ∼45 ka, which align thick crust zones with the ∼100 ka periods of lower sea level. Crustal thickness is also directly compared with seafloor bathymetry, indicating a component of compensated topography with RMS relief at the seafloor of 10 to 29% of crustal thickness variations. However, complexity inherited from variable asymmetric spreading and seafloor faulting is also apparent, and results provide new insights into how the crustal accretion filter modulates the recording of magma supply variations in the crust and in seafloor relief.
While the significance of the statistical analysis of these ridge records is limited by the short duration of the available crustal thickness dataset and effects of asymmetric spreading, the novel observations of crustal thickness varying at timescales of ∼80-100 ka require a mechanism, and the sea level hypothesis provides a plausible explanation.
Mid-ocean ridges are valuable archives of sedimentary flux records used to investigate atmospheric, oceanographic, and solid Earth responses to climate variability. Constant flux proxies, such as extraterrestrial helium-3 (3HeET) and excess thorium-230 (230ThXS), constrain vertical mass accumulation rates independent of the biases associated with lateral sediment transport and age model resolution. However, thorium scavenging by hydrothermal particles can perturb local 230ThXS deposition and complicate its application as a constant flux proxy in near-ridge environments. We characterize the footprint of hydrothermal scavenging on sedimentary 230ThXS using coupled 3HeET-230ThXS analyses in cores from the Mid-Atlantic Ridge and the Juan de Fuca Ridge. Samples deposited >10 km from the Juan de Fuca Ridge indicate reliable off-axis behavior of both constant flux proxies. In contrast, samples deposited <10 km from the Juan de Fuca Ridge axis and within the axial valley of the Mid-Atlantic Ridge suggest 50–80% deficits in sedimentary 230ThXS relative to its production rate. These deficits contrast with sedimentary 230ThXS surpluses recently observed on the East Pacific Rise. The spatial footprint of hydrothermal scavenging varies globally and temporally, likely as a function of the intensity of local hydrothermal activity. The combined ridge data suggest that near-vent sediments (typically within ∼5 km, but variable by ridge) receive relatively high 230ThXS deposition rates as a direct result of hydrothermal particle scavenging, while more distal sediments receive relatively low 230ThXS deposition rates due to diffusive loss of overlying seawater 230ThXS towards the vent. Aside from the East Pacific Rise, far-field sediments are likely to exhibit typical 230ThXS deposition rates at distances greater than ∼10 km of the ridge axis. However, 230ThXS systematics within the axial valleys of slow-spreading ridges may be complicated by other factors. Combined 3HeET-230ThXS studies at multiple ridges are needed to further characterize the nature of hydrothermal scavenging and to evaluate the potential of sedimentary 230ThXS anomalies to record large-scale variability in past hydrothermal activity.
New trace element abundances and isotope compositions for more than 100 mid‐ocean ridge basalts from 5.5°N to 19°N on the East Pacific Rise show step function variations in isotopic composition along the ridge axis that coincide with ridge discontinuities. Transform faults, overlapping spreading centers, and devals (deviation from axial linearity) mark the separation of individual clusters of distinct isotopic composition and trace element ratios that indicate source variations. This correlated chemical clustering and morphological segmentation indicates that source composition and segmentation can be closely related even on a fine scale. Substantial chemical variations within a segment are related to source composition. This suggests that even within segments the magma transport is mainly vertical, and there is limited along‐ridge transport, and there is little evidence for magma chambers that are well mixed along strike. Trace element concentrations show good correlations with isotopic compositions on a segment scale but less so on a regional scale. The trace element and isotopic variability along the northern East Pacific Rise can be explained by three mantle components: a depleted peridotite endmember, an enriched peridotite endmember, and a recycled gabbro‐like component. The gabbroic component has an isotopic signature indicating an ancient origin. The high‐resolution sampling indicates that within a segment the chemical variability is largely binary but that the endmembers of the binary mixing change from segment to segment. The endmembers of the binary variation within a segment are a combination of three of the endmembers.
The mantle sources of mid-ocean ridge basalts beneath the Indian and Pacific oceans have distinct isotopic compositions with a long-accepted boundary at the Australian–Antarctic Discordance along the Southeast Indian Ridge. This boundary has been widely used to place constraints on large-scale patterns of mantle flow and composition in the Earth’s upper mantle. Sampling between the Indian and Pacific ridges, however, has been lacking, especially along the remote 2,000 km expanse of the Australian–Antarctic Ridge. Here we present Sr, Nd, Hf and Pb isotope data from this region that show the Australian–Antarctic Ridge has isotopic compositions distinct from both the Pacific and Indian mantle domains. These data define a separate Zealandia–Antarctic domain that appears to have formed in response to the deep mantle upwelling and ensuing volcanism that led to the break-up of Gondwana 90 million years ago, and currently persists at the margins of the Antarctic continent. The relatively shallow depths of the Australian–Antarctic Ridge may be the result of this deep mantle upwelling. Large offset transforms to the east may be the boundary with the Pacific domain.
Vanadium isotope compositions of igneous rocks have the potential to constrain variations of physico-chemical conditions such as oxidation states during magmatism. Here, we present V isotope data for 27 fresh lavas (ranging from basaltic to dacitic compositions) from mid-ocean ridges, 31 altered basalts and gabbros from IODP site 1256 near the East Pacific Rise (EPR), and 2 back arc basin basalts (BABB). Our analyses of fresh mid-ocean ridge basalt (MORB) provide new constraints on the V isotope composition of MORBs, i.e. ‰δ51V=−0.84±0.02‰ (2SE, n=22). In addition, the mean δ51V of MORBs from individual segments is correlated with the mean ridge depth and Na8.0 of the segment, which might reflect the effect of melting extent on V isotope fractionation during mantle melting.
The mafic profile of intact altered oceanic crust (AOC) from the IODP site 1256 has δ51V ranging from −1.01 to ‰−0.77‰, similar to that of fresh MORBs, suggesting that V isotope fractionation is limited during alteration of oceanic crust. These results also indicate the V isotopic homogeneity of the bulk oceanic crust with average δ51V of ‰−0.85±0.02‰ (2SE, n=53), which is unaffected by ocean water and hydrothermal fluid alteration. Our results provide a guideline for application of V isotopes into studies of low and high temperature geochemical processes.
The evolved lavas (basaltic andesites, andesites, and dacites) from the East Pacific Rise (EPR) show apparent shifts towards heavy δ51V values with increasing degree of differentiation, which can be explained by the crystal–liquid fractionation during crystallization with an inferred isotope fractionation factor of Δ51Vmineral-melt=−0.15×106/T2. The enrichment of 51V with increasing differentiation degree for the 9°N Overlapping Spreading Center (OSC) lavas is consistent with direction of the isotope shift observed in lavas from Anatahan Island (Northern Mariana Arc) and Hekla Volcano (Iceland), but the magnitude (0.3‰) is much smaller than that (2‰) reported in Prytulak et al. (2017). Modeling of V isotope fractionation between mineral and melt shows that variations in redox condition are important for controlling V isotope fractionation, but insufficient to explain the dramatically different Δ51Vmineral-melt between 9°N OSC lavas and Anatahan/Hekla suites. More studies are necessary for better understanding of mechanisms of V isotope fractionation during magmatism.
Northwest Zhejiang Province (NWZJ) is located in the southeastern Lower Yangtze River Belt, southeastern China. Here we document the occurrence of both magnesian (149–131 Ma) and ferroan (162–121 Ma) granitoids in NWZJ. The magnesian granitoids are calc-alkalic peraluminous in composition, with a wide range of SiO2(58–72 wt%) contents. They have high K2O/Na2O, Sr/Y, and (La/Yb)N ratios, with insignificant Eu anomalies, whereas the calc-alkalic peraluminous ferroan granites have high SiO2 (76–77 wt%) contents, Fe indices (FeO∗/(FeO∗+MgO)FeO*/(FeO*+MgO)), and Ga/Al ratios. The ferroan granites also have low Ce/Pb and Nb/U ratios, with strong Ba, Sr, and Eu negative anomalies. Most of the rocks have similar zircon Lu-Hf isotopes (εHf(t)=−6.0εHf(t)=−6.0 to −0.7). However, rocks from two ferroan granitic bodies (Huangshitan and Jiuligang) have more depleted Hf isotopes, with εHf(t) ranging from −1.9 to 5.9. The whole-rock Nd isotopes of the ferroan granites (εNd(t)=−6.5εNd(t)=−6.5 to −3.2) are slightly more depleted than those of magnesian granitoids (εNd(t)=−8.8εNd(t)=−8.8 to −5.1). In addition, all ferroan granites show similar and high present-day whole-rock Pb isotopic ratios (18.3–18.8 for 206Pb/204Pb, 15.6–15.7 for 207Pb/204Pb, and 38.5–39.0 for 208Pb/204Pb). On the basis of published data and our new results, we propose that the magnesian granitoids were generated by partial melting of lower-crustal materials, whereas the ferroan granites were derived from a similar source but some more-depleted materials were added into their source after ∼135 Ma. The water contents of the magma may have played an important role in determining the different geochemical affinities of the felsic magmatism. The felsic magmatism occurred under an extensional setting during the period 162–121 Ma. The extension of the lithosphere was further enhanced and followed by upwelling of asthenospheric mantle after ∼135 Ma. This study suggests that a change in the tectonic regime occurred at ∼135 Ma in NWZJ, which may have been triggered by the rollback of the subducted Paleo-Pacific Plate.
Changes in meridional overturning circulation and water mass chemistry can be recorded by oxygen concentrations in the deep ocean. Because the deep Pacific is the largest ocean reservoir, its oxygen concentrations may be related to global climate change. In this study, oxygen conditions in the past are reconstructed by contrasting the sedimentary geochemistry of multiple redox-sensitive trace elements (Mn, Ni, Zn, V corrected for terrigenous and hydrothermal inputs) and authigenic U (aU) from six sediment cores on the Juan de Fuca Ridge from 2.7-2.8 km depth. We find that Mn and Ni are indicators for oxygen-rich conditions, while Zn, V, and aU are indicators for oxygen-poor conditions. Relative Redox Potentials (RRPs) for each core are calculated by converting excess metal fluxes into binary presence/absence designations, weighting each element by the strength and direction of its redox indication, summing the five elements, and then averaging the data in 5kyr bins. Metal depositional histories from all six cores demonstrate low oxygen conditions during interglacial periods, particularly during 100–120 ka (MIS5) but also 200–250 ka (MIS7), and high oxygen conditions during glacial periods (MIS2-4 and MIS6). This redox pattern does not appear to be driven by organic matter flux to the sediment, as reconstructed by three different paleo-productivity proxies (organic carbon, opal, and excess barium). Instead higher oxygen concentrations on the Juan de Fuca Ridge may be a result of better ventilation during glacial periods, possibly due to enhanced North Pacific Intermediate Waterformation. Alternatively, sedimentary redox conditions on the Juan de Fuca Ridge may be locally controlled by the deposition of hydrothermal sulfides from nearby vent fields.
The concentration of carbon in primary mid‐ocean ridge basalts (MORBs), and the associated fluxes of CO2 outgassed at ocean ridges, is examined through new data obtained by secondary ion mass spectrometry (SIMS) on 753 globally distributed MORB glasses. MORB glasses are typically 80–90% degassed of CO2. We thus use the limited range in CO2/Ba (81.3 ± 23) and CO2/Rb (991 ± 129), derived from undegassed MORB and MORB melt inclusions, to estimate primary CO2 concentrations for ridges that have Ba and/or Rb data. When combined with quality‐controlled volatile‐element data from the literature (n = 2,446), these data constrain a range of primary CO2 abundances that vary from 104 ppm to 1.90 wt%. Segment‐scale data reveal a range in MORB magma flux varying by a factor of 440 (from 6.8 × 105 to 3.0 × 108 m3/year) and an integrated global MORB magma flux of 16.5 ± 1.6 km3/year. When combined with CO2/Ba and CO2/Rb‐derived primary magma CO2 abundances, the calculated segment‐scale CO2 fluxes vary by more than 3 orders of magnitude (3.3 × 107 to 4.0 × 1010 mol/year) and sum to an integrated global MORB CO2 flux of × 1012 mol/year. Variations in ridge CO2 fluxes have a muted effect on global climate; however, because the vast majority of CO2degassed at ridges is dissolved into seawater and enters the marine bicarbonate cycle. MORB degassing would thus only contribute to long‐term variations in climate via degassing directly into the atmosphere in shallow‐water areas or where the ridge system is exposed above sea level.
Plain Language Summary
Estimated CO2 contents of primary mid‐ocean ridge basalts (MORB), calculated on a segment‐by‐segment basis, vary from 104 ppm to 1.9 wt%. CO2‐enriched MORB are present in all ocean basins, in particular, in the Atlantic Ocean basin, which is younger and more likely to contain admixed material from recent subduction compared to the much older Pacific Ocean basin. CO2 fluxes at individual ridge segments vary by 3 orders of magnitude due primarily to large variability in primary CO2 content. This study provides the most detailed and accurate estimate to date of the integrated total flux of CO2 from mid‐ocean ridges of × 1012 mol/year.
Sedimentary records of dust deposition in the subtropical Atlantic provide important constraints on millennial- and orbital-scale variability in atmospheric circulation and North African aridity. Constant flux proxies, such as extraterrestrial helium-3, yield dust flux records that are independent of the biases caused by lateral sediment transport and limited resolution that may be associated with age-model-derived mass accumulation rates. However, Atlantic dust records constrained using constant flux proxies are sparsely distributed and generally limited to the past 20 ka. Here we extend the Atlantic record of North African dust deposition to 70 ka using extraterrestrial helium-3 and measurements of titanium, thorium, and terrigenous helium-4 in two sediment cores collected at 26°N and 29°N on the Mid-Atlantic Ridge and compare results to model estimates for dust deposition in the subtropical North Atlantic. Dust proxy fluxes between 26°N and 29°N are well correlated, despite variability in lateral sediment transport, and underscore the utility of extraterrestrial helium-3 for constraining millennial-scale variability in dust deposition. Similarities between Mid-Atlantic dust flux trends and those observed along the Northwest African margin corroborate previous interpretations of dust flux variability over the past 20 ka and suggest that long distance transport and depositional processes do not overly obscure the signal of North African dust emissions. The 70 ka Mid-Atlantic record reveals a slight increase in North African dustiness from Marine Isotope Stage 4 through the Last Glacial Maximum and a dramatic decrease in dustiness associated with the African Humid Period. On the millennial-scale, the new records exhibit brief dust maxima coincident with North Atlantic cold periods such as the Younger Dryas, and multiple Heinrich Stadials. The correlation between Mid-Atlantic dust fluxes and previous constraints on North African aridity is high. However, precipitation exerts less control on dust flux variability prior to the African Humid Period, when wind variability governs dust emissions from consistently dry dust source regions. Thus, the Mid-Atlantic dust record supports the hypothesis that both aridity and wind strength drive dust flux variability across changing climatic conditions.
The coupled 100,000 year variations in ice volume, temperature, and atmospheric CO2 during the late Pleistocene are generally considered to arise from a combination of orbital forcing, ice dynamics, and ocean circulation. Also previously argued is that changes in glaciation influence atmospheric CO2 concentrations through modifying subaerial volcanic eruptions and CO2 emissions. Building on recent evidence that ocean ridge volcanism responds to changes in sea level, here it is suggested that ocean ridges may play an important role in generating late-Pleistocene 100 ky glacial cycles. If all volcanic CO2 emissions responded immediately to changes in pressure, subaerial and ocean-ridge volcanic emissions anomalies would oppose one another. At ocean ridges, however, the egress of CO2 from the mantle is likely to be delayed by tens-of-thousands of years, or longer, owing to ascent time. A simple model involving temperature, ice, and CO2 is presented that oscillates at ∼100 ky time scales when incorporating a delayed CO2 contribution from ocean ridge volcanism, even if the feedback accounts for only a small fraction of total changes in CO2. Oscillations readily become phase-locked with insolation forcing associated with changes in Earth's orbit. Under certain parameterizations, a transition from ∼40 ky to larger ∼100 ky oscillations occurs during the middle Pleistocene in response to modulations in orbital forcing. This novel description of Pleistocene glaciation should be testable through ongoing advances in understanding the circulation of carbon through the solid earth.
The composition of the convecting asthenospheric mantle that feeds the mantle wedge can be investigated via rear-arc lavas that have minimal slab influence. This “ambient mantle wedge” composition (the composition of the wedge prior to the addition of a slab component) varies substantially both worldwide and within individual arcs. 143Nd/144Nd measurements of rear-arc samples that have minimal slab influence are similar to 143Nd/144Nd in the stratovolcanoes of the adjacent volcanic fronts, suggesting that 143Nd/144Nd of arc-front volcanics are largely inherited from the ambient mantle composition. 143Nd/144Nd correlates with ratios such as Th/U, Zr/Nb, and La/Sm, indicating that these ratios also are strongly influenced by ambient wedge heterogeneity. The same phenomenon is observed among individual volcanoes from the Chilean Southern Volcanic Zone (SVZ), where along-strike variability of the volcanic front tracks that of rear-arc monogenetic volcanics. Depleted mantle wedges are more strongly influenced by slab-derived components than are enriched wedges. This leads to surprising trace element correlations in the global dataset, such as between Pb/Nb and Zr/Nb, which are not explicable by variable compositions or fluxes of slab components. Depleted ambient mantle is present beneath arcs with back-arc spreading; relatively enriched mantle is present adjacent to continents. Ambient mantle wedge heterogeneity both globally and regionally forms isotope mixing trajectories for Sr, Nd and Hf between depleted mantle and EM1-type enriched compositions as represented by Gough Island basalts. Making use of this relationship permits a quantitative match with the SVZ data. It has been suggested that EM1-type mantle reservoirs are the result of recycled lower continental crust, though such models do not account for certain trace element ratios such as Ce/Pb and Nb/U or the surprisingly homogeneous trace element compositions of EM1 volcanics. A model in which the EM1 end-member found in continental arcs is produced by low-degree melt-metasomatism of the sub-continental lithospheric mantle may be more plausible. The 143Nd/144Nd maximum along the SVZ may be a consequence of either rifting and collision of two ancient lithospheric domains or a slab tear. The correspondence of mantle wedge variations with EM1 suggests a potential role for metasomatized sub-continental lithosphere in creating EM1 sources globally.
Hydrothermal systems play an important role in modern marine chemistry, but little is known about how they may have varied on 100,000 year timescales. Here we present high-resolution records of non-lithogenic metal fluxes within sediment cores covering the last 500,000 years of hydrothermal deposition on the flanks of the Juan de Fuca Ridge. Six adjacent, gridded cores were analyzed by x-ray fluorescence for Fe, Mn, and Cu concentrations, corrected for lithogenic inputs with Ti, and normalized to excess initial 230Th to generate non-lithogenic metal flux records that provide the longest orbitally resolved reconstructions of hydrothermal activity currently available. Fe fluxes vary with global sea level over the last two glacial cycles, suggesting higher hydrothermal deposition during interglacial periods. The observed negative relationship between Fe and Mn indicates variable sediment redox conditions and diagenetic remobilization of sedimentary Mn over time. Thus, Mn fluxes may not be a reliable indicator for hydrothermal activity in the Juan de Fuca Ridge sediment cores. Cu fluxes show substantial high-frequency variability that may be linked to changes in vent temperature related to increased magmatic production during glacial periods. Deglacial hydrothermal peaks on the Juan de Fuca Ridge are consistent with previously published records from the Mid-Atlantic Ridge and the East Pacific Rise. Moreover, on the Juan de Fuca Ridge, the deglacial peaks in hydrothermal activity are followed by relatively high hydrothermal fluxes throughout the ensuing interglacial periods relative to the previous glacial period.
Sedimentation near mid-ocean ridges may differ from pelagic sedimentation due to the influence of the ridges' rough topography on sediment deposition and transport. This study explores whether the near-ridge environment responds to glacial-interglacial changes in climate and oceanography. New benthic δ18O, radiocarbon, multi-sensor track, and physical property (sedimentation rates, density, magnetic susceptibility) data for seven cores on the Juan de Fuca Ridge provide multiple records covering the past 700,000years of oceanographic history of the Northeast Pacific Ocean. Systematic variations in sediment density and coarse fraction correspond to glacial-interglacial cycles identified in benthic δ18O, and these observations may provide a framework for mapping the δ18O chronostratigraphy via sediment density to other locations on the Juan de Fuca Ridge and beyond. Sedimentation rates generally range from 0.5 to 3cm/kyr, with background pelagic sedimentation rates close to 1cm/kyr. Variability in sedimentation rates close to the ridge likely reflects remobilization of sediment caused by the high relief of the ridge bathymetry. Sedimentation patterns primarily reflect divergence of sedimentation rates with distance from the ridge axis and glacial-interglacial variation in sedimentation that may reflect carbonate preservation cycles as well as preferential remobilization of fine material.