Geochemical and physical sources of radon variation in a subterranean estuary — implications for groundwater radon activities in submarine groundwater discharge studies

Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 110 (2008): 120-127, doi:10.1016/j.marchem.2008.02.011.Submarine groundwater discharge (SGD), in form of springs and diffuse seepage, has long been recognized as a source of chemical constituents to the coastal ocean. Because groundwater is two to four orders of magnitude richer in radon than surface water, it has been used as both a qualitative and a quantitative tracer of groundwater discharge. Besides this large activity gradient, the other perceived advantage of radon stems from its classification as noble gas; that is, its chemical behavior is expected not to be influenced by salinity, redox, and diagenetic conditions present in aquatic environments. During our three-year monthly sampling of the subterranean estuary (STE) in Waquoit Bay, MA, we found highly variable radon activities (50-1600 dpm L-1) across the fresh-saline interface of the aquifer. We monitored pore water chemistry and radon activity at 8 fixed depths spanning from 2 to 5.6 m across the STE, and found seasonal fluctuations in activity at depths where elevated radon was observed. We postulate that most of pore water 222Rn is produced from particle-surface bound 226Ra, and that the accumulation of this radium is likely regulated by the presence of manganese (hydr)oxides. Layers of manganese (hydr)oxides form at the salinity transition zone (STZ), where water with high salinity, high manganese, and low redox potential mixes with fresh water. Responding to the seasonality of aquifer recharge, the location of the STZ and the layers with radium enriched manganese (hydr)oxide follows the seasonal land- or bay-ward movement of the freshwater lens. This results in seasonal changes in the depth where elevated radon activities are observed. The conclusion of our study is that the freshwater part of the STE has a radon signature that is completely different from the STZ or recirculated sea water. Therefore, the radon activity in SGD will depend on the ratio of fresh and recirculated seawater in the discharging groundwater.This work is a result of research sponsored by NSF (OCE- 0425061 to M.A.C.) and the WHOI Postdoctoral Scholar program (to H.D.)

Assessment of groundwater discharges into West Neck Bay, New York, via natural tracers

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 29 (2006): 1971-1983, doi:10.1016/j.csr.2006.07.011.A field experiment to compare methods of assessing submarine groundwater discharge (SGD) was held on Shelter Island, NY, in May 2002. We evaluated the use of radon, radium isotopes, and methane to assess SGD rates and dynamics from a glacial aquifer in the coastal zone. Fluxes of radon across the sediment-water interface were calculated from changes in measured surface water inventories following evaluation and correction for tidal effects, atmospheric evasion, and mixing with offshore waters. These fluxes were then converted to SGD rates using the measured radon concentration in the groundwater. We used the short-lived radium isotopes to calculate a horizontal mixing coefficient to assess radon loss by mixing between nearshore and offshore waters. We also made an independent calculation of SGD using the Ra-derived mixing coefficient and the long-lived 226Ra concentration gradient in the bay. Seepage rates were calculated to range between 0 and 34 cm.day-1 using the radon measurements and 15 cm.day-1 as indicated by the radium isotopes. The radiotracer results were consistent and comparable to SGD rates measured directly with vented benthic chambers (seepage meters) deployed during this experiment. These meters indicated rates between 2 and 200 cm.day-1 depending on their location. Both the calculated radon fluxes and rates measured directly by the automated seepage meters revealed a clear reproducible pattern of higher fluxes during low tides. Considering that the two techniques are completely independent, the agreement in the SGD dynamics is significant. Methane concentration in groundwater was very low (~30 nM) and not suitable as SGD tracer at this study site.The SGD intercomparison experiment was partially funded by SCOR, LOICZ, and UNESCO (IOC and IHP). W. C. Burnett acknowledges support from CICEET (Grant# 1368-810-41) and ONR (Grant# 1368-769-27). J. P. Chanton acknowledges support from Seagrant (R\C-E-44). The WHOI researchers acknowledge funding from CICEET (#NA07OR0351, NA17OZ2507)

pH-Dependent iron oxide precipitation in a subterranean estuary

Iron-oxide-coated sediment particles in subterranean estuaries can act as a geochemical barrier (biron curtainQ) for various chemical species in groundwater (e.g. phosphate), thus limiting their discharge to coastal waters. Little is known about the factors controlling this Fe-oxide precipitation. Here, we implement a simple reaction network in a 1D reactive transport model (RTM), to investigate the effect of O2 and pH gradients along a flow-line in the subterranean estuary ofWaquoit Bay (Cape Cod, Massachusetts) on oxidative precipitation of Fe(II) and subsequent PO4 sorption. Results show that the observed O2 gradient is not the main factor controlling precipitation and that it is the pH gradient at the mixing zone of freshwater (pH 5.5) and seawater (pH 7.9) near the beach face that causes a ~7-fold increase in the rate of oxidative precipitation of Fe(II) at ~15 m. Thus, the pH gradient determines the location and magnitude of the observed iron oxide accumulation and the subsequent removal of PO4 in this subterranean estuary

Sea level controls sedimentation and environments in coastal caves and sinkholes

Quaternary climate and sea-level research in coastal karst basins (caves, cenotes, sinkholes, blueholes, etc.) generally focuses on analyzing isotopes in speleothems, or associating cave elevations prior sea-level highstands. The sediments in coastal karst basins represent an overlooked source of climate and sea-level information in the coastal zone, but to accurately interpret these sediments first requires an understanding of the forcing mechanisms that emplace them. In this study, we hypothesize that coastal karst basins transition through vadose, littoral, anchialine, and finally into submarine environments during sea-level rise because groundwater and sea level oscillate in near synchrony in the coastal zone, causing each environment to deposit a unique sedimentary facies. To test this hypothesis, the stratigraphy in twelve sediment cores from a Bermudian underwater cave (Green Bay Cave) was investigated and temporally constrained with twenty radiocarbon dates. The results indicate that we recovered the first succession spanning the entire Holocene from an underwater cave (~ 13 ka to present). The sediments were characterized with X-radiography, fossil remains, bulk organic matter, organic geochemistry (δ13Corg, C:N), and grain size analysis. Four distinct facies represent the four depositional environments: (i) vadose facies (> 7.7 ka, calcite rafts lithofacies), (ii) littoral facies (7.7 to 7.3 ka: calcite rafts and mud lithofacies), (iii) anchialine facies (7.3 to 1.6 ka: slackwater and diamict lithofacies), and (iv) submarine facies (< 1.6 ka: carbonate mud and shell hash lithofacies). The onset and duration of these sedimentary depositional environments are closely linked to Holocene sea-level rise in Bermuda, indicating that sea level controls environmental development in coastal karst basins. Finally, we present a conceptual model for interpreting the sediments and environments in coastal karst basins as a result of sea-level change

Contrasting P-T-t paths reveal a metamorphic discontinuity in the New Quebec Orogen: insights into Paleoproterozoic orogenic processes

The large scale Paleoproterozoic Trans-Hudson Orogen exposed in Canada is recognized to record one of the first complete Wilson cycle. Although it is considered as a prototype of modern accretionary orogen, the strain pattern, thermal state, and architecture in its wedge are still poorly constrained. Part of it, the accretionary New Quebec Orogen (NQO, 1.82–1.77 Ga) resulted from oblique collision between the Superior Craton lower plate and a composite upper plate including the Kuujjuaq Domain (KD) and the Core Zone, and exposes supracrustal sequences of the Rachel-Laporte Zone (RLZ), a continental forearc basin that evolved in a collisional foreland basin upon collision. We performed an integrated analysis, using petrography, phase equilibria modelling and Lu-Hf garnet, and U-Pb zircon, monazite and rutile chronology on supracrustal rocks in the central segment of the New Quebec Orogen. On the one hand, the RLZ experienced a tight clockwise metamorphic evolution, with a peak at 650 °C/0.7 GPa. Prograde garnet and monazite growth is estimated at 1804 ± 8 Ma and 1796 ± 4 Ma respectively. On the other hand, the metamorphic peak for restitic granulites from the KD is estimated at 790 °C/0.72 GPa and follows isothermal decompression. Geochronology yields ages of 1836 ± 5 Ma for prograde garnet growth, and 1807 ± 4 Ma for crystallization of zircon from anatectic melt, which was followed by rapid cooling recorded by rutile at 1798 ± 16 Ma. We propose that the Lac Turcotte Fault that separates both domains has acted as a tectonometamorphic discontinuity that limits contrasting P-T-t paths. We suggest sequential burial of supracrustal sequences along an apparently continuous Barrovian sequence, with progressive recycling of inner zones in the foreland basin and subsequent involvement of the latter in the orogenic wedge.Antoine Godet, Carl Guilmette, Loic Labrousse, Matthijs A. Smit, Donald W.Davis, Tom Raimondo ... et al

Radium-228 as a tracer of dissolved trace element inputs from the Peruvian continental margin

International audienceContinental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured long-lived radium isotopes (Ra-226, Ra-228) along a zonal transect at 12 degrees S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used Ra-228 to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated Ra-228 activities were measured in surface water over the entire transect (similar to 8500 km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled Ra-228 shelf fluxes combined with water column dissolved TEI/Ra-228 ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 x 10(3) mu mol m(-2) y(-1)), DFe (1.5 x 10(3) mu mol m(-2) y(-1)), and DCo (1.0 x 10(2) mu mol m(-2) y(-1)). Second, co-occurring plumes of Ra-228, DFe, and DMn extended over 1800 km from the margin at 1000-2500 m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The Ra-228 gradient allowed us to derive an effective horizontal eddy diffusion coefficient (K-h) of 46 m(2) s(-1), which in turn permitted the calculation of slope sediment DMn (6.4 mu mol m(-2) y(-1)) and DFe (5.9 x 10(2) mu mol m(-2) y(-1)) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0-20 degrees S, the DMn shelf flux is approximately 2-3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is similar to 3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements

SUB-GAUSSIAN PROPERTY OF POSITIVE GENERALIZED WIENER FUNCTIONS

A real random variable X is sub-Gaussian iff there exist $K>0$ such that $E[ exp ( lambda X) ] leqq exp (K^2 \times lambda^22)$ for any $lambda in R$. J-P. Kahane [10] proved that a real random variable $X$ is sub-Gaussian if and only if $E[X]=0$ and $E[ exp ( \varepsilon X^2)]< infty$ for some $\varepsilon>0$. A probability measure $mu$ on a Banach space $B$ is said to be sub-Gaussian iff there exists $C>0$ such that int_B exp()mu(dx) leqq exp( \fraq{C^2}{2} int^2 mu(dx)) < infty for any $y in B^*$. (1) A Gaussian measure and the probability measure induced by a Rademacher series are typical examples of sub-Gaussian measures, and for these two probability measures, $exp( \varepsilon Arrowvert x^2 Arrowvert )$ is integrable for some $epsilon>0$ ([3], [12]). We call this integrability the exponential square integrability. When $B=L_p$ $(p geqq 1)$, (1) is a sufficient condition for the exponential square integrability, but not necessary even if $B$ is a Hilbert space ([4]). For a sub-Gaussian measure $mu$, the $L_p( mu )$ topologies $(0<p<infty)$ and the $L_0(μ)$ topology coincide on the family ${<y,x>;y in B^*}$. To show that considerably many probability measures satisfy such a remarkable property, we shall propve the sub-Gaussian property of probability measures for two types. One is a probability measure identified with a positive generalized Wiener function (see H. Sugita [17]), and the other is a probability measure which is absolutely continuous with respect to the probability measure induced by a random Fourier series. The former is exponentially square integrable [17], and so is the latter under certain additional conditions