Henrietta N. Edmonds

Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry

We have developed techniques by sector-field inductively coupled plasma mass spectrometry (ICP-MS) for measuring the isotopic composition and concentration of uranium and thorium, focusing on the rare isotopes, 230Th and 234U. These isotopes have been widely used as tracers in earth sciences, e.g., chronology, paleoclimatology, archeology, hydrology, geochemistry, and oceanography. Measurements made on reference materials demonstrate that the analytical precision approximates counting statistics and that the accuracy of the measurement is within error of accepted values. Routine measurement times are 20 min for U and 10 min for Th. The sensitivities (ions counted/atoms introduced) are 2–3‰ for U and 1.5–2‰ for Th. Samples of 10–40 ng of 238U (0.5–2.0 pg of 234U) give measurement precisions of 1–2‰ (2σ) for δ234U and U concentration ([U]). Only 0.4 pg of 230Th are needed to achieve [230Th] and 230Th/232Th data with errors less than 5‰ even for cases where 230Th/232Th is 10−5 or less. Our ICP-MS data, including uranium standards, thorium standards, 238U–234U–230Th–232Th dating of speleothems and 230Th–232Th in oceanic particulates, replicates measurements made by thermal ionization mass spectrometry (TIMS). Compared to TIMS, the ICP-MS method allows smaller sample size and higher sample throughput due to higher sensitivity, fewer sample preparation steps and shorter measurement times. However, mass biases, intensity biases, spectral interferences and instrumental blanks are significant and must be addressed.

Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge, Arctic Ocean

A high-resolution mapping and sampling study of the Gakkel ridge was accomplished during an international ice-breaker expedition to the high Arctic and North Pole in summer 2001. For this slowest-spreading endmember of the global mid-ocean-ridge system, predictions were that magmatism should progressively diminish as the spreading rate decreases along the ridge, and that hydrothermal activity should be rare. Instead, it was found that magmatic variations are irregular, and that hydrothermal activity is abundant. A 300-kilometre-long central amagmatic zone, where mantle peridotites are emplaced directly in the ridge axis, lies between abundant, continuous volcanism in the west, and large, widely spaced volcanic centres in the east. These observations demonstrate that the extent of mantle melting is not a simple function of spreading rate: mantle temperatures at depth or mantle chemistry (or both) must vary significantly along-axis. Highly punctuated volcanism in the absence of ridge offsets suggests that first-order ridge segmentation is controlled by mantle processes of melting and melt segregation. The strong focusing of magmatic activity coupled with faulting may account for the unexpectedly high levels of hydrothermal activity observed.

Discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean

Submarine hydrothermal venting along mid-ocean ridges is an important contributor to ridge thermal structure, and the global distribution of such vents has implications for heat and mass fluxes from the Earths crust and mantle and for the biogeography of vent-endemic organisms. Previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow-spreading ridges (ridges with full spreading rates <2 cm yr-1—which make up 25 per cent of the global ridge length), and that such vent systems would be hosted in ultramafic in addition to volcanic rocks. Here we present evidence for active hydrothermal venting on the Gakkel ridge, which is the slowest spreading (0.6–1.3 cm yr-1) and least explored mid-ocean ridge. On the basis of water column profiles of light scattering, temperature and manganese concentration along 1,100 km of the rift valley, we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites. Our discovery of such abundant venting, and its apparent localization near volcanic centres, requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow-spreading ridges.

Dissolved and particulate 231Pa and 230Th in the Atlantic Ocean: constraints on intermediate/deep water age, boundary scavenging, and 231Pa/230Th fractionation

231Pa and 230Th concentrations were determined in filtered seawater and suspended particulate matter collected from the Labrador Sea and the Equatorial and South Atlantic to constrain their application as tracers of intermediate/deep water age and Atlantic thermohaline circulation. Distributions of total 231Pa and 230Th indicate the influence of recently formed North Atlantic Deep Water, as evidenced by nearly invariant concentrations below ∼1000 m in the Labrador Sea and increasing 231Pa and 230Th concentrations as deep waters progress southward from northern source regions. Application of a scavenging–mixing model to both tracer distributions indicates an intermediate/deep water age of 12 yr in the Labrador Sea and a ∼30–140 yr transit time to the low-latitude stations. We attribute a striking increase in total 230Th in the Labrador Sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the Labrador Sea since 1993. The temporal change in the 230Th age of these waters is consistent with the 6 yr time interval between the observations. The average particulate 231Pa/230Th activity ratio in the Labrador Sea and low-latitude deep waters is 0.057±0.003, significantly below the 231Pa/230Th production ratio (0.093) and in agreement with excess 231Pa/230Th ratios in Holocene sediments (0.060±0.004) and sediment trap material (0.034±0.012) from the Atlantic and model simulations. This observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the Atlantic. A latitudinal dependence in particle fractionation of these tracers is also evident, with elevated fractionation factors (FTh/Pa) observed near the Equator and South Atlantic gyre (∼11) compared to low values in the Labrador Sea (∼3) and Southern Ocean (∼2). There also exists a depth dependence in FTh/Pa, characterized by low values in surface waters, a broad mid-depth maximum, and decreasing values towards the sea-floor. The latitudinal and depth variations in FTh/Pa are suggested to reflect differences in the chemical composition of marine particles.

Constraints on deep water age and particle flux in the equatorial and South Atlantic Ocean based on seawater 231Pa and 230Th data

High-precision measurements of 231 Pa and 230 Th in filtered seawater and suspended particulate matter samples are reported for the Equatorial and South Atlantic. Distributions of 231 Pa and 230 Th clearly indicate the influence of advection, as evidenced by departures from scavenging models that predict a linear increase with depth for these tracers. Application of a scavenging-mixing model implies a deep water transit time of ∼60-100 years from the northern source water regions. The average particulate 231 Pa/ 230 Th activity ratio is 0.0498 ± 0.0160, a factor of ∼2 lower than the 231 Pa/ 230 Th production ratio of 0.093 and in agreement with reported excess 231 Pa/ 230 Th ratios of 0.06 ± 0.004 in Holocene sediments north of 50°S in the Atlantic. These water column data further suggest that lateral eddy diffusive transport combined with enhanced scavenging in high-particle flux marginal regions (boundary scavenging) is weakly expressed in the Atlantic. Particle fractionation of these tracers is also indicated by the elevated fractionation factors of F Th/Pa = 4.32-24.04 (ave. = 9.97 ± 4.98) compared to values of ∼1-4 in the Southern Ocean.

Hydrothermal exploration of the Fonualei Rift and Spreading Center and the Northeast Lau Spreading Center

We report evidence for active hydrothermal venting along two back-arc spreading centers of the NE Lau Basin: the Fonualei Rift and Spreading Center (FRSC) and the Northeast Lau Spreading Center (NELSC). The ridge segments investigated here are of particular interest as the potential source of a mid-water hydrothermal plume (1500–2000 m depth) which extends more than 2000 km across the SW Pacific Ocean dispersing away from an apparent origin close to the most northeastern limits of the Lau Basin. Our results indicate the presence of at least four new hydrothermal plume sources, three along the FRSC and one on the NELSC, the latter situated within 150 km of the maximum for the previously identified SW Pacific regional-scale plume. However, TDFe and TDMn concentrations in the southernmost FRSC plume that we have identified only reach values of 19 and 13 nmol/L and dissolved 3He anomalies in the same plume are also small, both in relation to the SW Pacific plume and to local background, which shows evidence for extensive 3He enrichment throughout the entire Lau Basin water column. Our results reveal no evidence for a single major point hydrothermal source anywhere in the NE Lau Basin. Instead, we conclude that the regional-scale SW Pacific hydrothermal plume most probably results from the cumulative hydrothermal output of the entire topographically restricted Lau Basin, discharging via its NE-most corner.

Past, present, and future: A science program for the Arctic Ocean linking ancient and contemporary observations of change through modeling

The Arctic Ocean is the missing piece for any global model. Records of processes at both long and short timescales will be necessary to predict the future evolution of the Arctic Ocean through what appears to be a period of rapid climate change. Ocean monitoring is impoverished without the long-timescale records available from paleoceanography and the boundary conditions that can be obtained from marine geology and geophysics. The past and the present are the key to our ability to predict the future.