Search

Lithologic differences give rise to the differential weatherability of the Earth’s surface and globally variable silicate weathering fluxes, which provide an important negative feedback on climate over geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore water samples with depth were collected from two ridgetops and at three sites along a slope transect in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to determine weathering mechanisms and to estimate weathering rates. Regolith formed on both lithologies is highly leached of most labile elements, although Mg and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and weathering fluxes were determined by integrating mass losses over the thickness of solid-state weathering fronts, and are therefore averages over the timescale of regolith development. Resulting long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9 x 10-14 mol m-2 s-1, illite: 2.1 x 10-14 mol m-2 s-1 and kaolinite: 4.0 x 10-14 mol m-2 s-1. Long-term weathering fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, despite higher abundances of several elements in the granitic regolith. Contemporary weathering fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over space and time between these two watersheds indicate significant lithologic control of chemical weathering mechanisms and rates.

Germanium–silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 μmol/mol, with individual primary mineral phases ranging between 0.5 and 7 μmol/mol. The ratios in the bulk saprolite are higher (∼3 μmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 μmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1–1.6 μmol/mol) primarily due to the residual accumulation of Ge-poor quartz. Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27–0.47 μmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite–bedrock interface. Mass-balance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite–bedrock interface. Higher pore water Ge/Si ratios (∼1.2 μmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.

Sr isotope data from soils, water, and atmospheric inputs in a small tropical granitoid watershed in the Luquillo Mountains of Puerto Rico constrain soil mineral development, weathering fluxes, and atmospheric deposition. This study provides new information on pedogenic processes and geochemical fluxes that is not apparent in watershed mass balances based on major elements alone. 87Sr/86Sr data reveal that Saharan mineral aerosol dust contributes significantly to atmospheric inputs. Watershed-scale Sr isotope mass balance calculations indicate that the dust deposition flux for the watershed is 2100 ± 700 mg cm−2 ka−1. Nd isotope analyses of soil and saprolite samples provide independent evidence for the presence of Saharan dust in the regolith. Watershed-scale Sr isotope mass balance calculations are used to calculate the overall short-term chemical denudation velocity for the watershed, which agrees well with previous denudation rate estimates based on major element chemistry and cosmogenic nuclides. The dissolved streamwater Sr flux is dominated by weathering of plagioclase and hornblende and partial weathering of biotite in the saprock zone. A steep gradient in regolith porewater 87Sr/86Sr ratio with depth, from 0.70635 to as high as 0.71395, reflects the transition from primary mineral-derived Sr to a combination of residual biotite-derived Sr and atmospherically-derived Sr near the surface, and allows multiple origins of kaolinite to be identified.

Several explanations have been proposed to account for the dramatic perturbation to Earth's carbon cycle that led to extreme warming, ocean acidification, mass extinction, and a major negative carbon isotope excursion (CIE) at the Paleocene–Eocene (P/E) boundary. One reason that the ultimate cause for this event remains ambiguous is the difficulty in directly testing these multiple hypotheses. We conducted a test of one proposed mechanism, the “wildfire hypothesis,” that suggests that combustion of a significant fraction of abundant Paleocene peat deposits within several thousand years could release enough CO2 to Earth's atmosphere to account for all of these observations. Soot and graphitic black carbon (hereafter collectively referred to as GBC), both combustion byproducts, are widely distributed (through smoke) by subaerial combustion, and are remarkably resistant to diagenetic degradation. Using samples provided by the Ocean Drilling Program, we measured GBC concentrations across the P/E boundary in two important Paleocene–Eocene Thermal Maximum (PETM) sections to test a prediction of the wildfire hypothesis, namely that the combustion of Paleocene organic matter at the onset of the PETM would produce a peak in sedimentary GBC. In the Bass River section (New Jersey margin), GBC concentrations are uniformly high (121–358 ppm), consistent with the core's continental margin setting, but there is no evidence of increased GBC flux at the PETM. Furthermore, carbon isotopic ratios of GBC display the negative CIE seen in PETM organic matter, indicating that this material was sourced from contemporaneous (P/E boundary) biomass, not geologically old carbon. At site 1210 (Shatsky Rise), GBC concentrations were uniformly below our method's detection limit (0.5 ppm), whereas the wildfire hypothesis would predict high GBC concentrations (∼ 3000) ppm accompanying the event. Together, these data appear to refute but cannot conclusively rule out biomass burning as the cause for the PETM event.

Germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes. Sequestration of Ge by clay minerals is thought to be the dominant mechanism responsible for Ge/Si fractionation during weathering. In some environments, Ge may be sequestered by iron-oxyhydroxides. We examine Ge behavior in soils along a well-characterized climate gradient on the Hawaiian island of Maui to clarify the relationship between Fe geochemistry and soil Ge / Si ratios. All of the sites are developed on basaltic lavas and tephras approximately 400 ky old, are highly Si-depleted, and have Ge / Si ratios higher than parent material. Sites experiencing less than ∼3300 mm rain/yr host oxidized, highly weathered basaltic soils, in which Fe is present as secondary Fe-oxyhydroxide minerals. In contrast, the wetter sites (> 3340 mm/yr) show extensive Fe reduction, mobilization, and loss. We show that Ge sequestration is independent of Fe redox behavior along the gradient, and thus precipitation of Fe-oxyhydroxides is not a major factor contributing to Ge / Si fractionation during weathering. Instead, elevated soil Ge / Si ratios reflect partitioning of Ge into secondary clay minerals, plus additional retention of Ge by refractory non-silicates.

Ge/Si and 87Sr/86Sr data from primary and secondary minerals, soil waters, and stream waters in a tropical granitoid catchment quantitatively reflect mineral alteration reactions that occur at different levels within the bedrock–saprolite–soil zone. Near the bedrock–saprolite interface, plagioclase to kaolinite reaction yields low Ge/Si and 87Sr/86Sr. Higher in the regolith column, biotite weathering and kaolinite dissolution drive Ge/Si and 87Sr/86Sr to high values. Data from streams at base flow sample the bedrock–saprolite interface zone, while at high discharge solutes are derived from upper saprolite–soil zone. Coupled Ge/Si and 87Sr/86Sr can be effective tools for quantifying the importance of specific weathering reactions, and for geochemical hydrograph separation.

We examine the link between organic matter degradation, anaerobic methane oxidation (AMO), and sulfate depletion and explore how these processes potentially influence dolomitization. We determined rates and depths of AMO and dolomite formation for a variety of organic-rich sites along the west African Margin using data from Ocean Drilling Program (ODP) Leg 175. Rates of AMO are calculated from the diffusive fluxes of CH4 and SO4, and rates of dolomite formation are calculated from the diffusive flux of Mg. We find that the rates of dolomite formation are relatively constant regardless of the depth at which it is forming, indicating that the diffusive fluxes of Mg and Ca are not limiting. Based upon the calculated log IAP values, log Ksp values for dolomite were found to narrowly range between −16.1 and −16.4. Dolomite formation is controlled in part by competition between AMO and methanogenesis, which controls the speciation of dissolved CO2. AMO increases the concentration of CO32− through sulfate reduction, favoring dolomite formation, while methanogenesis increases the pCO2 of the pore waters, inhibiting dolomite formation. By regulating the pCO2 and alkalinity, methanogenesis and AMO can regulate the formation of dolomite in organic-rich marine sediments. In addition to providing a mechanistic link between AMO and dolomite formation, our findings provide a method by which the stability constant of dolomite can be calculated in modern sediments and allow prediction of regions and depth domains in which dolomite may be forming.

We present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the Ge/Si system as a tracer of silicate weathering processes in both modern and ancient environments. Intensely weathered soils developed on Hawaiian basalts have bulk soil Ge/Si ratios 2 to 10 times higher than fresh basalt (e.g., 10 to 25 μmol/mol vs. 2.5 μmol/mol). Soil Ge concentrations increase with Si, and decrease with Fe, suggesting that Ge sequestration is related to accumulation of secondary soil silicates, rather than retention in soil Fe oxy-hydroxides. Sequential extractions of these soils suggest that Ge/Si fractionation takes place by Ge sequestration during the initial precipitation of secondary soil aluminosilicates (principally allophane). Further Si loss and changes in mineralogy as these soils age result in little additional Ge/Si fractionation. Ge/Si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals. Kaolinite has a Ge/Si ratio (5.9 μmol/mol) higher than the plagioclase from which it forms (3.1 μmol/mol), whereas accumulation of primary quartz (Ge/Si 0.5 μmol/mol) prevents granitic soils from attaining high Ge/Si ratios. Laboratory synthesis of allophane confirms that Ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution.

Hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs, including sea salt aerosols and Asian dust. We use soil mineralogy and radiogenic isotope geochemistry (Sr and Nd) to evaluate the impact of Asian dust on a chronosequence of Hawaiian soils. Dust becomes an important constituent of soils 20 ky and older. Near-surface (4% of the total Nb contained in this soil profile.

Refractory trace element concentrations in strongly weathered Hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent-rock values due to extensive mass loss of more soluble major elements during pedogenesis. Nb and Ta exhibit virtually no mobility. Soil Nb/Ta ratios are within the range of fresh bedrock even when soil Nb concentrations are residually enriched by a factor of 10. In contrast, Al, Zr, and Hf are depleted relative to Nb in surface soil horizons but are enriched at depth, clearly indicating mobility of these elements. Variations in Th/Nb ratios in soil profiles indicate significant Th mobility within the soil column. However, mass-balance calculations require that accretion of Th-enriched Asian dust has resulted in a net increase in Th in some soils. Soils developed on a 150 ka rainfall gradient show that the mobility and loss of Zr increase with mean annual precipitation.

Modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium/silicon (Ge/Si) ratios compared to regions with less extensive chemical weathering, thought to be the result of Ge fractionation in 2:1 clays. Temporal variations in Ge/Si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering. However, this relationship has not been adequately ground-truthed by a terrestrial-based paleoclimate study utilizing Ge/Si ratios in lake diatoms. To this end, a sediment core was extracted from Dry Lake (el. 2763 m) in the headwaters of the Santa Ana River of southern California in July 1996. The Dry Lake drainage basin is comprised of biotite–muscovite gneiss and granite, with sparse pine forests and relatively high relief. The core had a basal AMS 14C age of 8,350±60 ybp. We successfully separated diatom samples large enough for cleaning, dissolution and chemical analysis without contamination by detrital materials (confirmed by trace metal analyses). Two dissolved opal samples were analyzed for Ge and Si concentrations along with modern water samples collected from waters within the drainage basin. Diatoms obtained from 8000-year-old sediments near the bottom of the core had an opal Ge/Si of 0.79×10−6 (mol/mol). A composite sample of diatoms deposited within the past 100 years yielded a significantly lower Ge/Si of 0.34×10−6. Analysis of Ge/Si was also performed from a variety of other materials in the Dry Lake basin, including stream and lake water, unweathered bedrock, soils, and mineral separates. Together with sedimentologic records from the lake sediments and Ge/Si recorded in other basin materials, it appears that the high Ge/Si values recorded in the 8000-year opal sample were the result of preferential early weathering of high Ge/Si biotite and muscovite minerals from the slopes. Thus, the detailed examination of Ge/Si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive Ge/Si paleorecords in some settings.

[1] We use dissolved silicon together with its “geochemical twin” germanium for the first time as a hydrologic tracer to study water delivery to the stream during storm events in the Rio Icacos watershed, Puerto Rico. Ge and Si were measured on base flow, stormflow, springwater, and soil water samples. Compositions of all of these waters appear to reflect varying contributions from three components, which we attribute to solutes released from bedrock weathering (groundwater), from short-term soil-water interaction (quick soil water), and longer-term soil-water interaction (matrix soil water). Base flow stream waters have high Si and moderate Ge (Ge/Si ratio ∼0.29 μmol/mol), consistent with a predominantly bedrock weathering source as indicated by their similarity with water sampled from springs emerging from the saprolite-bedrock boundary on a hillslope landslide scar. During storm events there is a shift toward more dilute compositions (but higher Ge/Si ratios) similar to those measured on water samples from temporary depression storage and overland flow (quick soil water). Geochemical mass balance shows that 80%–90% of the stream chemistry can be explained by mixing groundwater with this quick soil water composition, which we infer to reflect new water traveling as shallow throughflow. Stream water δ18O values decrease to more negative values typical of precipitation supporting rapid delivery of rainwater to the stream channel during stormflow. The third component, with a Ge-rich composition characteristic of soil matrix water sampled by tension lysimeters, is required to explain higher stream water Ge/Si ratios measured during hydrograph recession. We infer from this an additional, slower, and less dominant pathway for delivery of soil water to the stream channel.

The past ~25 m.y. of geologic history in the northern ~300 km (~33°–36°S) of the Andean Southern Volcanic Zone has seen waxing and waning magmatic production rates and episodic eastward relocation of arc segments accompanied by abrupt chemical changes in the magmas. These changes can be linked to episodes of crustal thickening at times of backarc thrusting and to peaks of subduction erosion of forearc crust and mantle lithosphere at times of frontal-arc migration to the east. The magmatic-tectonic coupling is well seen in the history—enhanced by 28 new K-Ar ages, >160 major and trace element analyses, and Sr, Nd, and Pb isotope analyses—of a west to east transect through the El Teniente copper district near 34°S. The temporal trends in magmatic chemistry in this transect are like the well-documented south to north trends in Pleistocene to Holocene volcanic centers of the Southern Volcanic Zone, and both can be linked to the same events. The magmatic changes require differences in magma source regions as shown by isotopic data and in depths of crustal magma generation/fractionation as shown by pressure-sensitive trace element distributions. “Adakitic” magmas in the region are attributed to a combination of melting the base of thickened lower crust and crust entering the mantle through subduction erosion. Subduction erosion is argued to peak in episodes of frontal-arc migration at ca. 19–16 Ma and ca. 7–4 Ma. The combined effects of crustal shortening and forearc truncation in the past 20 m.y. near 34°S have led to the loss of ~170 km of crustal width. The timing and arc length over which these events occurred show that subduction of the Juan Fernandez Ridge on the Nazca plate cannot have been the major driving force. The history of the region shows the importance of non-steadystate processes in arc-magma production and the necessity of studying arc systems over millions, not tens of thousands, of years.