ALBERT

Description: In the caption of Figure 1, panel D, the longitude is incorrect. The correct longitude is 120.1058°W. See PDF file for details.

Description: Splitting of the Vitiaz arc formed the Tonga-Kermadec and Lau-Colville Ridges (southwestern Pacific Ocean), separated by the Lau Basin in the north and Havre Trough in the south. We present new trace element and Sr-Nd-Hf-Pb isotope geochemistry for the Kermadec and Colville Ridges extending ~900 km north of New Zealand (36°S–28°S) in order to (1) compare the composition of the arc remnants with Quaternary Kermadec arc volcanism, (2) constrain spatial geochemical variations in the arc remnants, (3) evaluate the effect of Hikurangi igneous plateau subduction on the geochemistry of the older arc lavas, and (4) elucidate what may have caused arc splitting. Compared to the Kermadec Ridge, the Colville Ridge has higher more-incompatible to less-incompatible immobile element ratios and largely overlapping isotope ratios, consistent with an origin through lower degrees of melting of more enriched upper mantle in the Vitiaz rear arc. Between ca. 8 and 3 Ma, both halves of the arc (~36°S–29°S) included a more enriched (EM1-type) composition (with lower 206Pb/204Pb and 207Pb/204Pb and higher Δ8/4 Pb [deviation of the measured 208Pb/204Pb ratio from a Northern Hemisphere basalt regression line] and 87Sr/86Sr) compared to older and younger arc lavas. High-Ti basalts from the Manihiki Plateau, once joined to the Hikurangi Plateau, could serve as the enriched Vitiaz arc end member. We propose that the enriched plateau signature, seen only in the isotope ratios of mobile elements, was transported by hydrous fluids from the western margin of the subducting Hikurangi Plateau or a Hikurangi Plateau fragment into the overlying mantle wedge. Our results are consistent with plateau subduction triggering arc splitting and backarc opening.

Description: Precambrian carbonates record secular variations in the style of CaCO3 nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO3 nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO4 above ~12 μmol/L (μM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO3 supersaturation (Ωcal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO3 mineralization pathways and sustain high CaCO3 supersaturation.

Description: Terrestrial methane (CH4) emissions may have increased during the Paleocene-Eocene Thermal Maximum (PETM; ca. 56 Ma) and promoted additional warming, especially in the high latitudes. Although there is evidence for increased CH4 cycling in a single Northern Hemisphere site, whether enhanced methane cycling was globally widespread is unknown because there have been no subsequent investigations. The mechanism of CH4 release is also unknown because a direct comparison between temperature and CH4 cycling has so far not been possible. Here we use biomarkers to reconstruct temperature change and CH4 cycling in a new PETM-aged succession in New Zealand. Our results indicate that the stable carbon isotopic composition (δ13C) of bacterial hopanoids decreased to very low values (–60‰) during the onset of the PETM, indicating enhanced consumption of CH4. These values are much lower than found in modern wetlands and suggest a major perturbation of the CH4 cycle during the onset of the PETM. Low hopanoid δ13C values do not persist into the early Eocene, despite evidence for elevated temperatures. This indicates that the terrestrial CH4 cycle operates differently during transient compared to gradual warming events. Enhanced CH4 cycling during the PETM may help to resolve the temperature data-model mismatch in the high latitudes and could yield higher estimates of Earth system sensitivity than expected from CO2 alone.

Description: Much of our understanding of early eukaryote diversity and paleoecology comes from the record of organic-walled microfossils in shale, yet the conditions controlling their preservation are not well understood. It has been suggested that high concentrations of total organic carbon (TOC) inhibit the preservation of organic fossils in shale, and although this idea is supported anecdotally, it has never been tested. Here we compared the presence, preservational quality, and assemblage diversity of organic-walled microfossils to TOC concentrations of 346 shale samples that span the late Paleoproterozoic to middle Neoproterozoic in age. We found that fossil-bearing samples have significantly lower median TOC values (0.32 wt%, n = 189) than those containing no fossils (0.72 wt%, n = 157). Preservational quality, measured by the loss of surface pattern, density of pitting, and deterioration of wall margin, decreases as TOC increases. Species richness negatively correlates with TOC within the ca. 750 Ma Chuar Group (Arizona, USA), but no relationship is observed in other units. These results support the hypothesis that high TOC content either decreases the preservational quality or inhibits the preservation of organic-walled microfossils altogether. However, it is also possible that other causal factors, including sedimentation rate and microbial degradation, account for the correlation between fossil preservation and TOC. We expect that as TOC varies in space and time, so too does the probability of finding well-preserved fossils. A compilation of 13,940 TOC values spanning Earth history suggests significantly higher median TOC levels in Mesoproterozoic versus Neoproterozoic shale, potentially biasing the interpreted pattern of increased eukaryotic diversity in the Tonian.

Description: Sedimentary basins are the archives of ancient environmental conditions on planetary surfaces, and on Mars they may contain the best record of surface water and habitable conditions. While erosional valley networks have been mapped, the global distribution of fluvial sedimentary deposits on Mars has been unknown. Here we generated an eight-trillion-pixel global map of Mars using data from the NASA Context Camera (CTX), aboard the Mars Reconnaissance Orbiter spacecraft, to perform the first systematic global survey of fluvial ridges—exhumed ancient deposits that have the planform shape of river channels or channel belts, but stand in positive relief due to preferential erosion of neighboring terrain. We used large fluvial ridges (〉70 m width) as a conservative proxy for the occurrence of depositional rivers or river-influenced depositional areas. Results showed that fluvial ridges are as much as 100 km long, common across the southern highlands, occur primarily in networks within intercrater plains, and are not confined to impact basins. Ridges were dominantly found in Noachian through Late Hesperian units, consistent with cessation of valley network activity, and occurred downstream from valley networks, indicating regional source-to-sink transport systems. These depositional areas mark a globally distributed class of sedimentary deposits that contain a rich archive of Mars history, including fluvial activity on early Mars.

Description: Along the present-day circum-Pacific subduction girdle, seamount subduction is known to have significant effects on subduction dynamics including on seismicity and arc magmatism. Because seamount subduction should have occurred throughout much of Earth history, its effects on orogenesis in the overriding plate should be identifiable in ancient orogens. In this study, we investigate the Paleozoic South Tianshan orogen of Central Asia, for which abundant evidence of seamount subduction exists, further bolstered by the continuation of a long-lived plume-induced large igneous province on the subsequently accreted Tarim craton. We find that semi-continuous seamount subduction from ca. 400 to 330 Ma temporarily shut down arc magmatism, and once the seamounts were completely subducted, then arc magmatism resumed and eclogites were quickly exhumed. If such an orogenic fingerprint of seamount subduction can be identified in ancient orogens, a much more complete picture of plume-subduction interaction and its influence on both crustal and mantle processes can be developed.

Description: Seabirds in the Canadian Arctic congregate in large colonies, producing oases of biological productivity and diversity in coastal regions. Here, we examined sterols, stanols, and stable isotopes (δ15N and δ13C) in three 14C-dated pond sediment cores near a large seabird colony and archaeological site on Devon Island (Nunavut, Canada), showing historical occupation by the seabirds and an ancient human (Thule or Norse) settlement over an ~1100 yr time period. Coprostanol in the sediment records captures the presence of humans at ca. 1150 CE, followed by their abandonment of the site by ca. 1300 CE. Increased seabird presence at this site after ca.1200 CE is indicated by increases in δ15N and cholesterol/sitosterol. Seabird population expansion is observed after ca. 1600 CE in δ15N and cholesterol/sitosterol profiles, coinciding with European whaling activities that expanded in the seventeenth through nineteenth centuries. Our study provides insights into human and seabird occupation in the High Arctic to inform archaeological and conservation efforts.

Description: Europe’s largest gas field, the Groningen field (the Netherlands), is widely known for induced subsidence and seismicity caused by gas pressure depletion and associated compaction of the sandstone reservoir. Whether compaction is elastic or partly inelastic, as implied by recent experiments, is a key factor in forecasting system behavior and seismic hazard. We sought evidence for inelastic deformation through comparative microstructural analysis of unique drill core recovered from the seismogenic center of the field in 2015, 50 yr after gas production started, versus core recovered before production (1965). Quartz grain fracturing, crack healing, and stress-induced Dauphiné twinning are equally developed in the 2015 and 1965 cores, with the only measurable effect of gas production being enhanced microcracking of sparse K-feldspar grains in the 2015 core. Interpreting these grains as strain markers, we suggest that reservoir compaction involves elastic strain plus inelastic compression of weak clay films within grain contacts.

Description: The presence of localized low-velocity anomalies in the upper mantle beneath the passive Atlantic margin in North America is a puzzling geophysical observation. Whether the anomalies are caused by the remnant heat from past hotspots or ongoing asthenospheric upwelling is still debated. We addressed the formation of the anomalies based on a recent velocity model for eastern North America, which reveals new information on their shapes and anisotropic signatures. The low-velocity anomaly in New England appears as a narrow column above 90 km depth and broadens westward at depths of 120–200 km. Two slow anomalies are imaged under the central Appalachian Mountains between 140 km and 240 km. These low velocities correspond to pronounced positive radial anisotropy (Vsh 〉 Vsv), indicating a dominantly horizontal asthenospheric flow. They also coincide with the tracks of the Great Meteor hotspot (140–115 Ma) and an inferred hidden hotspot (60–50 Ma). The anomalies in the central Appalachians could be due to lithospheric interaction with the second hotspot and subsequent lithospheric instabilities. The complex shape of the New England anomaly is consistent with interaction with both hotspots. The first hotspot could have eroded the base of the lithosphere, forming a channel, and the second hotspot could have further thinned the lithosphere and produced a localized cavity at shallow depths. Consequently, the indented lithosphere base would have filled with channelized asthenospheric flow or produced small-scale convection, helping to sustain the slow anomaly. Low-velocity anomalies at the North America passive margin are likely the consequences of prior hotspot interactions.