Bruce K. Nelson

The SrCa-temperature relationship in coralline aragonite: Influence of variability in (SrCa)seawater and skeletal growth parameters

This paper provides an evaluation of two of the most likely pitfalls of Sr/Ca thermometry, i.e., the effect of biogenic cycling of Sr vs. Ca in the surface ocean and the effect of variable extension rate on Sr incorporation in coralline aragonite. The authors also report calibration of the Sr/Ca-temperature relationship for three coral species, Porites lobata, Pocillopora eydouxi, and Pavona clavus, collected for the Hawaiian and Galapagos islands. Analyses of seawater samples show significant spatial and depth variability in the Sr:Ca ratio. The uncertainty introduced by this effect is estimated to be <0.2[degrees]C for corals located in tropical oligotrophic waters, and potentially larger for corals located in upwelling areas. Sr/Ca along two different growth axes of a Galapagos Pavona clavus, with annual extension rates of [approximately]6 and 12 mm/y, respectively, indicate an offset of 1-2[degrees]C, with higher Sr/Ca values associated with slower extension rates. The offset observed between the two growth axes may be the result of variations in extension and/or calcification rate. These results are important in determining past sea surface temperatures for reconstruction of paleoclimates.

Biological Controls on Coral Sr/Ca and δ18O Reconstructions of Sea Surface Temperatures

Coral strontium/calcium ratios have been used to infer that the tropical sea surface temperature (SST) cooled by as much as 6�C during the last glacial maximum. In contrast, little or no change has been inferred from other marine-based proxy records. Experimental studies of the effect of growth rate and the magnitude of intraspecific differences indicate that biological controls on coral skeletal strontium/calcium uptake have been underestimated. These results call into question the reliability of strontium/calcium-based SST reconstructions.

Uranium in foraminiferal calcite as a recorder of seawater uranium concentrations

We present results of an investigation of uraniumcalcium ratios in cleaned foraminiferal calcite as a recorder of seawater uranium concentrations. For accurate reconstruction of past seawater uranium content, shell calcite must incorporate uranium in proportion to seawater concentration and must preserve its original uranium composition over time. Laboratory culture experiments with live benthic (Amphistegina lobifera) and live planktonic (Globigerinella calida) foraminifera show that the UCa ratio of cleaned calcite tests is proportional to the concentration of uranium in solution. After correcting results for the presence of initial calcite, the apparent distribution coefficient D = (UCa)calciteUCa)solution = 10.6 ± 0.3 (×10−3) for A. lobifera and D = 7.9 ± 0.1 (×10−3) for G. calida. UCa ratios in planktonic foraminifera from core tops collected above 3900 m in the equatorial Atlantic and above 2100 m in the Pacific Ocean show no significant difference among the species analyzed. D estimated from core top samples ranges from 7.6 ± 0.4 (× 10−3) for O. universa to 8.4 ± 0.5 (×10−3) for G. ruber. In benthic species C. wuellerstorfi, D = 7.0 ± 0.8 (×10−3). UCa and Mg/Ca in G. tumida and G. sacculifer from core tops taken near and below the regional lysocline decrease with water depth. Smaller decreases in UCa and MgCa with depth were observed in C. wuellerstorfi. In the planktonic species, we believe that UCa and MgCa are lower in the more dissolution-resistant fraction of calcite, leading to lower UCa in more highly dissolved samples.

Petrology of the Chilliwack batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH2O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH2O<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH2O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; εNd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; εNd(0)=+3.3 to +6.9). The fH2O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.

Assimilation of felsic crust by basaltic magma: Thermal limits and extents of crustal contamination of mantle-derived magmas

Basaltic magma is the most widely used probe of mantle chemistry and structure. Chemical characterization of the sources for mantle-derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks. The most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization (AFC). Because of the energetic link between these processes, predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma–country-rock system. Using the MELTS software package, we modeled isenthalpic (heat-balanced) AFC between two common types of basaltic magma and several types of felsic crust at 1 kbar. Our simulations show that during the early stages of isenthalpic AFC, the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (2.0–2.7), allowing assimilation of a mass of country rock up to 5%–18% of the initial magma mass with only 3%–7% crystallization. The second stage of AFC, beginning with plagioclase and/or pyroxene saturation, is characterized by lower r (0.5–1.0). The initial high- r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses. Under certain conditions, even small degrees of crystallization of olivine alone, coupled with cooling of the magma, can accommodate relatively large amounts of crustal assimilation, and cause large shifts in isotopic and trace element geochemical indices with little differentiation.

The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico)

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SLC41A2 encodes a plasma-membrane Mg2+ transporter

The TRPM7 (transient receptor potential melastatin 7) ion channel has been implicated in the uptake of Mg2+ into vertebrate cells, as elimination of TRPM7 expression through gene targeting in DT40 B-lymphocytes renders them unable to grow in the absence of supplemental Mg2+. However, a residual capacity of TRPM7-deficient cells to accumulate Mg2+ and proliferate when provided with supplemental Mg2+ suggests the existence of Mg2+ uptake mechanism(s) other than TRPM7. Evaluation of the expression of several members of the SLC41 (solute carrier family 41) family, which exhibit homology with the MgtE class of prokaryotic putative bivalent-cation transporters, demonstrated that one, SLC41A2 (solute carrier family 41 member 2), is expressed in both wild-type and TRPM7-deficient DT40 cells. Characterization of heterologously expressed SLC41A2 protein indicated that it is a plasma-membrane protein with an N-terminus-outside/C-terminus-inside 11-TM (transmembrane)-span topology, consistent with its functioning as a trans-plasma-membrane transporter. In contrast with a previous report of ion-channel activity associated with SLC41A2 expression in oocytes, investigation of whole cell currents in SLC41A2-expressing DT40 cells revealed no novel currents of any type associated with SLC41A2 expression. However, expression of SLC41A2 in TRPM7-deficient cells under the control of a doxycycline-inducible promoter was able to conditionally enhance their net uptake of 26Mg2+ and conditionally and dose-dependently provide them with the capacity to grow in the absence of supplemental Mg2+, observations strongly supporting a model whereby SLC41A2 directly mediates trans-plasma-membrane Mg2+ transport. Overall, our results suggest that SLC41A2 functions as a plasma-membrane Mg2+ transporter in vertebrate cells.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Temporal-compositional-isotopic trends in rejuvenated-stage magmas of Kauai, Hawaii, and implications for mantle melting processes

Primitive, low-silica and high-alkali magmas that erupt late in the evolution of most ocean island volcanoes are highly enriched in incompatible trace elements, yet their isotopic compositions require a time-integrated mantle source history of incompatible-element depletion. Reconciling these observations has traditionally required either extremely low degrees of partial melting of depleted mantle (commonly less than 0.2%), invoking an unusual mantle source recently enriched in incompatible elements or extensive melt-mantle interaction. Analyses of stratigraphic sequences of rejuvenated-stage volcanics of Kauai, Hawaii show previously unrecognized isotopic-trace element correlations, as well as temporal variations within monogenetic lava sequences that provide evidence of variable degrees of melting of several distinct mantle sources. Inter-element and isotopic-trace-element correlations indicate little or no chromatographic effects on melt compositions, inconsistent with the expected effects of significant melt-mantle reaction as the source of their incompatible-element enrichment. Trace-element compositions of rejuvenated-stage magmas can be produced by melting of typical depleted mantle sources only if they are mixtures of small- and large-degree (0.1% and 2–15%, depending on source mineralogy) melts of isotopically distinct sources. The simplest model for the Koloa Volcanics, however, consistent with previous interpretations of other Hawaiian lavas, is that they are derived from a range of incompatible-element enriched mantle sources variably metasomatized by small-degree melts of depleted mantle. Isotopic-trace-element trends in the Koloa magmas (of the opposite sense as the overall Hawaiian trend) are best explained by a positive correlation between the extent of source metasomatism and degree of melting to produce the Koloa magmas. Systematic decreases in incompatible element concentrations within individual eruption sequences probably represent sequential eruption of progressively larger-degree melt, possibly caused by vertical zonation in extent of melting in the source regions, or extraction of low-degree melts from surrounding mantle by early-ascending magma batches.

Vanadium in foraminiferal calcite: Evaluation of a method to determine paleo-seawater vanadium concentrations

We assess the potential of using foraminiferal calcite as a paleoceanographic indicator of seawater V concentrations. Laboratory culture experiments show that living benthic and planktonic foraminifera incorporate V into their test in direct proportion to seawater concentrations. Distribution coefficients relative to the culture solution are D = 2.1 × 10−3 and 2.8 × 10−3 for G. calida and A. lobifera, respectively. We use a cleaning procedure that effectively removes most V-rich contaminant phases from foraminiferal calcite preserved in the fossil record including organic matter and MnFe oxyhydroxides. MnCO3 overgrowths cannot be eliminated. Since V is conservative in the ocean, foraminiferal calcite recently accreted and found in surface sediment samples should have the same V content if this tracer accurately reflects seawater concentrations. V /Ca values for the same species of foraminifera are constant for core-top samples collected above the foraminiferal lysocline in different ocean basins. The mean distribution coefficients relative to seawater are D = 5.8 (± 1.0) × 10−3, 10.3 (±0.7) × 10−3 and 32 (+2.5) X 10−3 for G. sacculifer, G. tumida, and C. wuellerstorfi, respectively. These differences suggest that V incorporation is species dependent. Core-top analyses along two submarine rises in the Equatorial Atlantic and Pacific oceans indicate significant dissolution effects. With increasing depth of deposition, and thus more extensive partial dissolution of the test, VCa decreases by a factor of three in G. tumida, increases by up to a factor of four in G. sacculifer, and increases by a factor of two in C. wuellerstorfi. No exchange between foraminiferal V and detrital V in sediments is observed over an interval of 200 kyr.