Determining the source(s) of hydrogen, carbon, and nitrogen accreted by Earth is important for understanding the origins of water and life and for constraining dynamical processes that operated during planet formation. Chondritic meteorites are asteroidal fragments that retain records of the first few million years of solar system history. The deuterium/hydrogen (D/H) values of water in carbonaceous chondrites are distinct from those in comets and Saturn's moon Enceladus, implying that they formed in a different region of the solar system, contrary to predictions of recent dynamical models. The D/H values of water in carbonaceous chondrites also argue against an influx of water ice from the outer solar system, which has been invoked to explain the nonsolar oxygen isotopic composition of the inner solar system. The bulk hydrogen and nitrogen isotopic compositions of CI chondrites suggest that they were the principal source of Earth's volatiles.

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We report data on the martian meteorite Northwest Africa (NWA) 7034, which shares some petrologic and geochemical characteristics with known martian meteorites of the SNC (i.e., shergottite, nakhlite, and chassignite) group, but also has some unique characteristics that would exclude it from that group. NWA 7034 is a geochemically enriched crustal rock compositionally similar to basalts and average martian crust measured by recent Rover and Orbiter missions. It formed 2.089 ± 0.081 billion years ago, during the early Amazonian epoch in Mars' geologic history. NWA 7034 has an order of magnitude more indigenous water than most SNC meteorites, with up to 6000 parts per million extraterrestrial H(2)O released during stepped heating. It also has bulk oxygen isotope values of Δ(17)O = 0.58 ± 0.05 per mil and a heat-released water oxygen isotope average value of Δ(17)O = 0.330 ± 0.011 per mil, suggesting the existence of multiple oxygen reservoirs on Mars.

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Seasonal variability in stable carbon (S'XZ) and nitrogen (b15N) isotope ratios was observed in suspended particulate matter of the Delaware estuary. Two major pools of organic matter were found in the estuary-phytoplankton growing in situ and a mixture of planktonic and terrestrial detritus. In general, the 6°C and 615N of suspended particulate matter reflected planktonic dominance. With the background chemical and physical information available for the estuary, it is evident that biogeochemical processes influence isotopic distributions in the estuary to a greater extent than does physical mixing. During spring, we postulate that isotopic fractionation of ammonium assimilated at concentrations >20 PM resulted in more negative 615N values for organic matter fixed by phytoplankton. As algal growth proceeded, the 615N of seston reached a maximum (+ 1 SY&) because phytoplankton were using a pool of NH,+ enriched in 15N as a result of previous fractionation during assimilation. Similarly, maximal 813C values were related to high rates of primary productivity associated with algal growth. Decreased isotopic fractionation occurred at high rates of production, implying that diffusion of CO, across the cell membrane became increasingly rate limiting.The 613C values in bottom sediments were equivalent to those in suspended particulate matter, but a 2Y60 difference in 615N was found between suspended and bottom sediments. With nitrogen isotopic differences between water-column seston and surficial sediments, we estimate that 15-30% of planktonic production is deposited in the sediments during spring. If this organic matter is remineralized in late summer and fall, it could support up to 20% of primary production at that time.Differences exist among natural abundances of stable carbon isotopes (813C) and stable nitrogen isotopes (615N) in organic matter from terrestrial and anthropogenic I Present address: Department of Oceanography, Texas A&M University, College Station 77843.