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K. McLaughlin

Publications and source records attributed to K. McLaughlin.

7 recordsLinked to original sources
USGS2009

Characterizing the oxygen isotopic composition of phosphate sources to aquatic ecosystems

The oxygen isotopic composition of dissolved inorganic phosphate (δ 18 O p ) in many aquatic ecosystems is not in isotopic equilibrium with ambient water and, therefore, may reflect the source δ 18 O p . Identification of phosphate sources to water bodies is critical for designing best management practices for phosphate load reduction to control eutrophication. In order for δ 18 O p to be a useful tool for source tracking, the δ 18 O p of phosphate sources must be distinguishable from one another; however, the δ 18 O p of potential sources has not been well characterized. We measured the δ 18 O p of a variety of known phosphate sources, including fertilizers, semiprocessed phosphorite ore, particulate aerosols, detergents, leachates of vegetation, soil, animal feces, and wastewater treatment plant effluent. We found a considerable range of δ 18 O p values (from +8.4 to +24.9‰) for the various sources, and statistically significant differences were found between several of the source types. δ 18 O p measured in three different fresh water systems was generally not in equilibrium with ambient water. Although there is overlap in δ 18 O p values among the groups of samples, our results indicate that some sources are isotopically distinct and δ 18 O p can be used for identifying phosphate sources to aquatic systems.

Environmental Science & Technology↗
USGS2009

Using oxygen isotopes of phosphate to trace phosphorus sources and cycling in lake Erie

Water samples collected during three sampling trips to Lake Erie displayed oxygen isotopic values of dissolved phosphate (δ 18 O p ) that were largely out of equilibrium with ambient conditions, indicating that source signatures may be discerned. δ 18 O p values in the Lake ranged from +10‰ to +17‰, whereas the equilibrium value was expected to be around +14‰. The riverine weighted average δ 18 O p value was +11‰ and may represent one source of phosphate to the Lake. The lake δ 18 O p values indicated that there must be one or more as yet uncharacterized source(s) of phosphate with a high δ 18 O p value. Potential sources other than rivers are not yet well-characterized with respect to δ 18 O of phosphate, but we speculate that a likely source may be the release of phosphate from sediments under reducing conditions created during anoxic events in the hypolimnion of the central basin of Lake Erie. Identifying potential phosphorus sources to the Lake is vital for designing effective management plans for reducing nutrient inputs and associated eutrophication.

Environmental Science & Technology↗
USGS2006

Phosphate oxygen isotope ratios as a tracer for sources and cycling of phosphate in North San Francisco Bay, California

[1] A seasonal analysis assesing variations in the oxygen isotopic composition of dissolved inorganic phosphate (DIP) was conducted in the San Francisco Bay estuarine system, California. Isotopic fractionation of oxygen in DIP (exchange of oxygen between phosphate and environmental water) at surface water temperatures occurs only as a result of enzyme‐mediated, biological reactions. Accordingly, if phospate demand is low relative to input and phosphate is not heavily cycled in the ecosystem, the oxygen isotopic composition of DIP (δ 18 O p ) will reflect the isotopic composition of the source of phosphate to the system. Such is the case for the North San Francisco Bay, an anthropogenically impacted estuary with high surface water phosphate concentrations. Variability in the δ 18 O p in the bay is primarily controlled by mixing of water masses with different δ 18 O p signatures. The δ 18 O p values range from 11.4‰ at the Sacramento River to 20.1‰ at the Golden Gate. Deviations from the two‐component mixing model for the North Bay reflect additional, local sources of phosphate to the estuary that vary seasonally. Most notably, deviations from the mixing model occur at the confluence of a major river into the bay during periods of high river discharge and near wastewater treatment outlets. These data suggest that δ 18 O p can be an effective tool for identifying P point sources and understanding phosphate dynamics in estuarine systems.

Journal of Geophysical Research G: Biogeosciences↗
USGS2006

Oxygen isotopes of phosphatic compounds - Application for marine particulate matter, sediments and soils

The phosphate oxygen isotopic composition in naturally occurring particulate phosphatic compounds (δ 18 O p ) can be used as a tracer for phosphate sources and to evaluate the cycling of phosphorus (P) in the environment. However, phosphatic compounds must be converted to silver phosphate prior to isotopic analysis, a process that involves digestion of particulate matter in acid. This digestion will hydrolyze some of the phosphatic compounds such that oxygen from the acid solution will be incorporated into the sample as these phosphatic compounds are converted to orthophosphate (PO 4 3− ). To determine the extent of incorporation of reagent oxygen into the sample, we digested various phosphatic compounds in both acid amended with H 2 18 O (spiked) and unspiked acid and then converted the samples to silver phosphate for δ 18 O p analysis. Our results indicate that there is no isotopic fractionation associated with acid digestion at 50 °C. Furthermore, we found that reagent oxygen incorporation is a function of the oxygen to phosphorus ratio (O:P) of the digested compound whereby the percentage of reagent oxygen incorporated into the sample is the same as that which is required to convert all of the P-compounds into orthophosphate. Based on these results, we developed a correction for reagent oxygen incorporation using simple mass balance, a procedure that allows for the determination of the δ 18 O p of samples containing a mixture of phosphatic compounds. We analyzed a variety of environmental samples for δ 18 O p to demonstrate the utility of this approach for understanding sources and cycling of P.

Marine Chemistry↗
USGS2004

A precise method for the analysis of δ18O of dissolved inorganic phosphate in seawater

A method for preparation and analysis of the oxygen isotope composition (δ 18 O) of dissolved inorganic phosphate (DIP) has been developed and preliminary results for water samples from various locations are reported. Phosphate is extracted from seawater samples by coprecipitation with magnesium hydroxide. Phosphate is further purified through a series of precipitations and resin separation and is ultimately converted to silver phosphate. Silver phosphate samples are pyrolitically decomposed to carbon monoxide and analyzed for δ 18 O. Silver phosphate samples weighing 0.7 mg (3.5 µmol oxygen) can be analyzed routinely with an average standard deviation of about 0.3‰. There is no isotope fractionation during extraction and blanks are negligible within analytical error. Reproducibility was determined for both laboratory standards and natural samples by multiple analyses. A comparison between filtered and unfiltered natural seawater samples was also conducted and no appreciable difference was observed for the samples tested. The δ 18 O values of DIP in seawater determined using this method range from 18.6‰ to 22.3‰, suggesting small but detectable natural variability in seawater. For the San Francisco Bay estuary DIP δ 18 O is more variable, ranging from 11.4‰ near the San Joaquin River to 20.1‰ near the Golden Gate Bridge, and was well correlated with salinity, phosphate concentration, and δ 18 O of water.

California↗