Louis A. Derry

Changing sources of nutrients during four million years of ecosystem development

As soils develop in humid environments, rock-derived elements are gradually lost, and under constant conditions it seems that ecosystems should reach a state of profound and irreversible nutrient depletion. We show here that inputs of elements from the atmosphere can sustain the productivity of Hawaiian rainforests on highly weathered soils. Cations are supplied in marine aerosols and phosphorus is deposited in dust from central Asia, which is over 6,000 km away.

Biological control of terrestrial silica cycling and export fluxes to watersheds

Silicon has a crucial role in many biogeochemical processes—for example, as a nutrient for marine and terrestrial biota, in buffering soil acidification and in the regulation of atmospheric carbon dioxide. Traditionally, silica fluxes to soil solutions and stream waters are thought to be controlled by the weathering and subsequent dissolution of silicate minerals. Rates of mineral dissolution can be enhanced by biological processes. But plants also take up considerable quantities of silica from soil solution, which is recycled into the soil from falling litter in a separate soil–plant silica cycle that can be significant in comparison with weathering input and hydrologic output. Here we analyse soil water in basaltic soils across the Hawaiian islands to assess the relative contributions of weathering and biogenic silica cycling by using the distinct signatures of the two processes in germanium/silicon ratios. Our data imply that most of the silica released to Hawaiian stream water has passed through the biogenic silica pool, whereas direct mineral–water reactions account for a smaller fraction of the stream silica flux. We expect that other systems exhibiting strong Si depletion of the mineral soils and/or high Si uptake rates by biomass will also have strong biological control on silica cycling and export.

Neogene Himalayan weathering history and river87Sr86Sr: impact on the marine Sr record

Abstract Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since > 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The87Sr86Sr ratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for87Sr86Sr in the paleo-GB basin. The Sr data from pedogenic clays shows that river87Sr86Sr values were near 0.72 prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned to ≤ 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of87Sr86Sr in a major river system. The high87Sr86Sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river87Sr86Sr values require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity,87Sr86Sr and Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater87Sr86Sr results from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya.

Organic carbon burial forcing of the carbon cycle from Himalayan erosion

Weathering and erosion can affect the long-term ocean–atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon. Recent attention has focused on increased silicate weathering of tectonically uplifted areas in the India–Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era. The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 consumption from the net burial of organic carbon during Himalayan sediment deposition was 2–3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes.

Refractory element mobility in volcanic soils

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.

The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh

Himalayan rivers have very unusual Sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic. We present Sr, O, and C isotopic data from river and rain water, bedload, and bedrock samples for the western and central Nepal Himalaya and Bangladesh, including the monsoon season. Central Himalayan rivers receive Sr from several sources: carbonate and clastic Tethyan sediments, High Himalayan Crystalline (HHC) gneisses and granitoids with minor marbles, carbonates and metasediments of the Lesser Himalaya (LH), and Miocene-Recent foreland basin sediment from the Siwaliks group and the modern flood plain. In the Tethyan Himalaya rivers have dissolved [Sr] ≈ 6 μmol/l and 87Sr/86Sr ≈ 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with [Sr] ≈ 0.2 μmol/l and 87Sr/86Sr ≈ 0.74. Lesser Himalayan streams also have low [Sr] ≈ 0.4 μmol/l and are highly radiogenic (87Sr/86Sr ≥ 0.78). Highly radiogenic carbonates of the LH do not contribute significantly to the Sr budget because they are sparse and have very low [Sr]. In large rivers exiting the Himalaya, Sr systematics can be modeled as a mixture between Tethyan rivers, where slightly radiogenic carbonates (mean 87Sr/86Sr ≈ 0.715) are the main source of Sr, and Lesser Himalaya waters, where extremely radiogenic silicates (>0.8) are the main source of Sr. HHC waters are less important because of their low [Sr]. Rivers draining the Siwaliks foreland basin sediments have [Sr] ≈ 4 μmol/l and 87Sr/86Sr ≈ 0.725. Weathering of silicates in the Siwaliks and the flood plain results in a probably significant radiogenic (0.72–0.74) input to the Ganges and Brahmaputra (G-B), but quantification of this flux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low [Sr] and high 87Sr/86Sr during the monsoon. Sr in the Brahmaputra ranges from 0.9 μmol/l and 0.722 in March to 0.3 μmol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 × 108 mol/yr with 87Sr/86Sr = 0.7295.

Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan

Abstract We report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period between c. 17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. ɛNd(0) values in the Bengal Fan sediments (all samples) define a narrow range about − 16.0. 87Sr/86Sr values (all samples) are also in a narrow range near 0.741. δ18O values in quartz separates define a narrow range at +12.8±0.5‰. Coarse biotite-chlorite separates give δ18O = 3.6−5.6‰. Combined δ18O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). δ18O in the IC clay fractions is 11.5–15‰, while SK clays are 18.2–22.6‰. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between δ18O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably <20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible. A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.

Accretion of Asian dust to Hawaiian soils: isotopic, elemental, and mineral mass balances

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 (<50-cm depth) horizons contain as much as 30% quartz, a mineral absent from local parent material. Basaltic Sr and Nd isotope signatures in these horizons have been completely overprinted by Asian dust signatures, with 87Sr/86Sr ratios as high as 0.723 and eNd values as low as −7. REE patterns in these soils are indistinguishable from that of average upper continental crust. Quartz abundance and Nd isotopes provide two independent tracers of long-term dust additions to the chronosequence soils. The two tracers indicate a minimum long-term average dust accretion rate of 125 mg cm−2 ky−1 at the 150 ka chronosequence site, roughly a factor of three higher than Holocene dust accumulation rates estimated from North Pacific sediment cores. We find that the mass of dust preserved in these soil profiles does not increase with age in soils 150 ka and older, requiring a loss mechanism for accreted dust. On the basis of the geomorphic stability of these sites, observed preferential loss of dust-derived mica relative to quartz, and estimates of soil Si leaching rates we argue that chemical weathering is the dominant loss mechanism for dust from these soils. Dust has a profound effect on the budgets of elements that are susceptible to leaching losses. Dust becomes the dominant source of soil nutrients Si and P in the oldest, most intensely weathered soils. We calculate a dust-derived P input flux of 0.8 mg cm−2 ky−1, and a dust-derived Si input flux of 35 mg cm−2 ky−1. Si leaching fluxes are high (1400 mg cm−2 ky−1) in the youngest (2 ka) soils and drop systematically with soil age to a value that closely balances the dust-derived Si input flux by 4100 ka. Extremely refractory elements such as Nb, which are concentrated in soils by residual enrichment processes, are much less readily impacted by dust addition. Although dust has had a pronounced impact on Sr and Nd isotopic budgets and on soil mineral composition at the 150 ka site, dust cannot have contributed >4% of the total Nb contained in this soil profile.

Changing sources of base cations during ecosystem development, Hawaiian Islands

Sr/ 86 Sr evidence from a soil chronosequence in the Hawaiian Islands demonstrates that the atmosphere supplies >85% of putatively rock-derived Sr in older sites. Initially, bedrock is the dominant source for Sr and other lithophile elements such as Ca, but high rates of weathering and leaching of the substrate by 20 ka lead to a shift to atmospheric sources. The loss of weathering inputs coincides with other physio-chemical changes in the soil and results in a steep decline of base cations in the soil pool. While these patterns imply the potential for limitation of biological productivity by low base cation supply, the atmosphere provides a supply of base cations in excess of nutritional needs, even after nearly all rock-derived base cations have been leached from the soil. This raises the possibility that P limitation in terrestrial ecosystems may develop at least as much because of low rates of atmospheric deposition of P (relative to Ca, K, and other rock- derived elements) as because of its chemical interaction in soil.

δ13C of organic carbon in the Bengal Fan: Source evolution and transport of C3 and C4 plant carbon to marine sediments

Carbon isotopic measurements on organic carbon (oc) in sediment cores from the Bengal Fan (ODP Leg 116) show a dramatic 10 %. increase beginning ca. 7 Ma ago, and a rapid decrease after 0.9 Ma. These shifts reflect changes in the mixing ratio of terrigenous carbon derived from C3 and C4 plants. The rapid increase in δ13C of Bengal Fan OC at 7 Ma shows that Late Miocene expansion of C4 plants already documented in the Siwaliks was widespread over all the Himalayan foreland. After 7 Ma, relations of δ13C with sedimentological parameters show that C4 plants dominate in the foreland whereas C3 plants remain abundant in the mountain range. Variations in the source of the sediments and of the OC appear to be sensitive to climate-hydrologic conditions in the basin. Major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass.