Trace-element and isotopic constraints on the source of magmas in the active volcano and Mariana island arcs, Western Pacific

Analytical results of the relative and absolute abundance of LIL-incompatible trace elements (K, Rb, Cs, Sr, and Ba) and isotopic compositions ( , , and ) are summarized for fresh samples from active and dormant volcanoes of the Volcano and Mariana island arcs. The presence of thickened oceanic crust (T 15–20 km) beneath the arc indicates that while hybridization processes resulting in the modification of primitive magmas by anatectic mixing at shallow crustal levels cannot be neglected, the extent and effects of these processes on this arc's magmas are minimized. All components of the subducted plate disappear at the trench. This observation is used to reconstruct the composition of the crust in the Wadati-Benioff zone by estimating proportions of various lithologies in the crust of the subducted plate coupled with analyses from DSDP sites. Over 90% of the mass of the subducted crust consists of basaltic Layers II and III. Sediments and seamounts, containing the bulk of the incompatible elements, make up the rest. Bulk Western Pacific seafloor has , δ ¹⁸O +7.2, K/Rb 510, K/Ba 46, and K/Cs 13,500. Consideration of trace-element data and combined systematics limits the participation of sediments in magmagenesis to less than 1%, in accord with the earlier results of Pb-isotopic studies. Combined data indicate little, if any, involvement of altered basaltic seafloor in magmagenesis. Perhaps more important than mean isotopic and LIL-element ratios is the restricted range for lavas from along over 1000 km of this arc. Mixtures of mantle with either the subducted crust or derivative fluids should result in strong heterogeneities in the sources of individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations of crustal materials supplied to the subduction zone; and (2) lesser efficiency of mixing processes accompanying induced convection between arc segments (parallel to the arc) as compared to that perpendicular to the arc. The absence of these heterogeneities indicates that either some process exists for the efficient mixing of mantle and subducted material parallel to the arc or that subducted materials play a negligible role in the generation of Mariana-Volcano arc melts.Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition of this arc's magmas, while (2) a mantle similar to that which has produced alkali-olivine basalts (AOB) of north Pacific “hot spot” chains is indistinguishable in many respects spects from the source of these arc lavas.     

Mid-Holocene Hydrologic Model of the Shingobee Watershed, Minnesota

A hydrologic model of the Shingobee Watershed in north-central Minnesota was developed to reconstruct mid-Holocene paleo-lake levels for Williams Lake, a surface-water body located in the southern portion of the watershed. Hydrologic parameters for the model were first estimated in a calibration exercise using a 9-yr historical record (1990–1998) of climatic and hydrologic stresses. The model reproduced observed temporal and spatial trends in surface/groundwater levels across the watershed. Mid-Holocene aquifer and lake levels were then reconstructed using two paleoclimatic data sets: CCM1 atmospheric general circulation model output and pollen-transfer functions using sediment core data from Williams Lake.Calculated paleo-lake levels based on pollen-derived paleoclimatic reconstructions indicated a 3.5-m drop in simulated lake levels and were in good agreement with the position of mid-Holocene beach sands observed in a Williams Lake sediment core transect. However, calculated paleolake levels based on CCM1 climate forcing produced only a 0.05-m drop in lake levels. We found that decreases in winter precipitation rather than temperature increases had the largest effect on simulated mid-Holocene lake levels. The study illustrates how watershed models can be used to critically evaluate paleoclimatic reconstructions by integrating geologic, climatic, limnologic, and hydrogeologic data sets.     

Formation rate of water masses in the Japan sea

Water masses in the subsurface and the intermediate layer are actively formed due to strong winter convection in the Japan Sea. It is probable that some fraction of pollution is carried into the layer below the sea surface together with these water masses, so it is important to estimate the formation rate and turnover time of water masses to study the fate of pollutants. The present study estimates the annual formation rate and the turnover time of water masses using a three-dimensional ocean circulation model and a particle chasing method. The total annual formation rate of water masses below the sea surface amounted to about 3.53 ± 0.55 Sv in the Japan Sea. Regarding representative intermediate water masses, the annual formation rate of the Upper portion of the Japan Sea Proper Water (UJSPW) and the Japan Sea Intermediate Water (JSIW) were estimated to be about 0.38 ± 0.11 and 1.43 ± 0.16 Sv, respectively, although there was little evidence of the formation of deeper water masses below a depth of about 1500 m in a numerical experiment. An estimate of turnover time shows that the UJSPW and the JSIW circulate in the intermediate layer of the Japan Sea with timescales of about 22.1 and 2.2 years, respectively.     

Stable isotope variations in modern tropical speleothems: Evaluating equilibrium vs. kinetic isotope effects

Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ¹⁸O and δ¹³C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ¹³C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The ¹³C depletions are likely caused by kinetically driven departures in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions, caused by rapid calcite growth. ¹³C enrichments can be accounted for by Rayleigh distillation of the HCO₃⁻ (aq) reservoir during degassing of ¹³C-depleted CO₂.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in ¹⁸O relative to equilibrium values. δ¹⁸O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ¹³C, δ¹⁸O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO₂ hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO₃⁻ (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in ¹⁸O enrichment in the HCO₃⁻ (aq) reservoir and ultimately in the precipitated CaCO₃.     

Transformation dynamics and reactivity of dissolved and colloidal iron in coastal waters

We have investigated the chemical forms, reactivities and transformation kinetics of Fe(III) species present in coastal water with ion exchange and filtration methods. To simulate coastal water system, a mixture of ferric iron and fulvic acid was added to filtered seawater and incubated for a minute to a week. At each incubation time, the seawater sample was acidified with hydrochloric acid and then applied to anion exchange resin (AER) to separate negatively charged species (such as fulvic acid, its complexes with iron and iron oxyhydroxide coated with fulvic acid) from positively charged inorganic ferric iron (Fe(III)′). By monitoring the acid-induced Fe(III)′ over an hour, it was found that iron complexed by fulvic acid dissociated rapidly to a large extent (86–92% at pH 2), whereas amorphous ferric oxyhydroxide particles associated with fulvic acid (AFO-L) dissociated very slowly with the first-order dissociation rate constants ranging from 6.1 × 10^− 5 for pH 3 to 2.7 × 10^− 4 s^− 1 for pH 2. Therefore, a brief acidification followed by the AER treatment (acidification/AER method) was likely to be able to determine fulvic acid complexes and thus differentiate the complexes from the AFO-L particles (the dissolution of AFO-L was insignificant during the brief acidification). The acidification/AER method coupled with a simple filtration technique suggested that the iron–fulvic acid complexes exist in both the < 0.02 μm and 0.02–0.45 μm size fractions in our coastal water system. The truly dissolved iron (< 0.02 μm) was relatively long-lived with a life-time of 14 days, probably due to the complexation by strong ligands. Such an acid-labile iron may be an important source of bioavailable iron in coastal environments, as a significant relationship between the chemical lability and bioavailability of iron has been well recognised.     

Multifractal analysis of earthquakes

Multifractal properties of the epicenter and hypocenter distribution and also of the energy distribution of earthquakes are studied for California, Japan, and Greece. The calculatedD _q-q curves (the generalized dimension) indicate that the earthquake process is multifractal or heterogeneous in the fractal dimension. Japanese earthquakes are the most heterogeneous and Californian earthquakes are the least. Since the earthquake process is multifractal, a single value of the so-called fractal dimension is not sufficient to characterize the earthquake process. Studies of multifractal models of earthquakes are recommended. Temporal changes of theD _q-q curve are also obtained for Californian and Japanese earthquakes. TheD _q-q curve shows two distinctly different types in each region; the gentle type and the steep type. The steeptype corresponds to a strongly heterogeneous multifractal, which appears during seismically active periods when large earthquakes occur.D _q for smallq or negativeq is considerably more sensitive to the change in fractal structure of earthquakes thanD _q forq2. We recommend use ofD _q at smallq to detect the seismicity change in a local area.