No abstract

A number of rivers have been found to transport highly aged organic matter [OM]; however, the sources of this aged material remain a matter of debate. One potential source may be erosion and weathering of headwater lithologies rich in ancient sedimentary OM. In this study, waters, suspended particulates, streambed sediments, rocks and soils from fourteen small headwater watersheds of a mid-size, temperate, passive margin river were sampled and characterized by D 14 C, d 13 C, and POC/TPN ratios to identify sources of particulate and dissolved OM delivered to the river mainstem. These headwater sites encompass a range in lithology (OM-rich shales, OM-lean carbonate/mudstone facies, and OM-free crystalline rocks) and land use types (forested and agricultural), and allow investigation of the influence of agriculture and bedrock types on stream OM characteristics. Streams draining large areas of both agricultural land use and OM-rich lithology contain particulate OM [POM] that is more 14 C-depleted than streams draining forested, shale-free watersheds. However, this is not sufficient to account for the significantly lower D 14 C-POC measured in the river mainstem. Dissolved OM [DOM] D 14 C are in all cases enriched compared to POM from the same stream, but are otherwise highly variable and unrelated to either land use or lithology. POC/TPN ratios were likewise highly variable. POC and DOC d 13 C signatures were similar across all watersheds. Based on isotope mass balance, 14 C-free fossil OM sources contribute 0-12% of total stream POM. Although these results do not unequivocally separate the influences of land use and lithology, watershed coverage by shale and agriculture are both important controls on stream D 14 C-POC. Thus export of aged, particle-associated OM may be a feature of river systems along both passive and active continental margins.

We report the deep distributions of noble gases, helium isotopes, and radiocarbon measured during the U.S. GEOTRACES GP16 East Pacific Zonal Transect between 152 and 77°W at 12-15°S in the South Pacific. The dominant feature is an intense tongue of hydrothermal effluent that extends more than 4,000 km westward from the East Pacific Rise (EPR) at ~2500m depth. The patterns reveal significant "downstream" variations in water mass structure, advection, and mixing that belie the simple perception of a continuous plume extending westward from the EPR. For example, one feature observed at 120°W, 14°S has tracer signatures that are consistent with a water mass originating from an area as much as 2,000 km south of this section, suggesting a quasi-permanent northward flow on the western flank of the EPR. Helium isotope variations in the plume show a uniquely high In the western end of the section, incoming bottom waters have relatively less hydrothermal hydrothermal helium, more radiocarbon, and more oxygen, as well as negative saturation anomalies for the heavy noble gases (Ar, Kr, and Xe). During the basin-scale upwelling of this water, diapycnal mixing serves to erase these negative anomalies. The relative magnitudes of the increases for the heavy noble gases (Ar, Kr, and Xe) are quantitatively consistent with this process. This leads us to estimate the relatively smaller effects on He and Ne saturations, which range from near zero to 0.2% and 0.3% respectively. With this information, we are able to refine our estimates of the magnitude of He ratio of non-atmospheric helium introduced into deep Pacific waters.pg. 2

a b s t r a c tWe present the distributions of helium isotopes (in the form of helium isotope ratio anomaly relative to the atmospheric ratio) and tritium along two sections occupied in the subtropical North Atlantic as part of the first U.S. GEOTRACES survey (GEOTRACES GA03). The general distributions of these isotopes are consistent with the continuing penetration and evolution of bomb-produced tritium and its daughter isotope 3 He in the main thermocline and along the western boundary current system. We combine these two distributions to compute a tritium-3 He age, which is related to the elapsed time since the water was at the ocean surface. Although it is an indicator biased by the effects of mixing and influenced by the time history and spatial distribution of bomb tritium delivery to the ocean surface, it still remains a useful measure of ventilation time-scales. Aside from the continued propagation of the tritium-3 He transient into the ocean interior, there are three notable features of interest in these distributions. The first is the clear signature of upwelling in the water column near the coast of Mauritania, characterized by the upward bowing of isochrones in the thermocline and discernable 3 He excess at the ocean surface. A simple 3 He mass balance calculation suggests an upwelling flux of order 1.8 Â 10 6 m 3 s À 1 (1.8 Sv) along the Mauritanian coast. The second is a mid-depth ($ 1500-2000 m) core of ventilated waters centered over the Mid-Atlantic Ridge, an anticyclonic circulation of waters likely originating in the Labrador Sea. The third notable feature is a volcanic 3 He plume at about 3500 m depth emanating from the TAG Hydrothermal Area that is detectable as much as 500 km away on each side of the Mid-Atlantic Ridge. We estimate a 3 He:heat ratio of $ 7 Â 10 -18 mol J À 1 and a 3 He flux from the TAG site of $ 15 mmol y À 1 . Since 3 He is a conserved tracer in the absence of measureable tritium, the correlation of volcanic 3 He with other hydrothermally influenced TEIs (e.g., Fe) can be used as a dilution tracer as probe of non-conservative behavior in the water column. Also, since the regional and global fluxes of volcanic 3 He are known, the correlations can be used as a regional/global flux gauge for hydrothermal input of those TEIs.

ABSTRACT.A new and unique radiocarbon accelerator mass spectrometry (AMS) facility has been constructed at the Woods Hole Oceanographic Institution. The defining characteristic of the new system is its large-gap optical elements that provide a larger-than-standard beam acceptance. Such a system is ideally suited for high-throughput, high-precision measurements of 14 C. Details and performance of the new system are presented.