Kevin M. Yeager

The spatial distribution and emission of nitrous oxide (N2O) in a large eutrophic lake in eastern China: anthropogenic effects.

The emission of N(2)O from China is globally significant, but relatively few direct observations have been made in many of the fresh water environments most likely to be important sites of N(2)O production. In this paper, N(2)O saturations were examined in the ecologically heterogeneous, eutrophied, Lake Taihu, as well as in surrounding rivers in eastern China. The emissions of N(2)O were estimated and compared with those from other landscapes within the Lake Taihu drainage basin. We found that anthropogenically-enhanced inorganic N inputs act as a limited primary control on the spatial distribution of N(2)O saturations in heavily eutrophied parts of the lake only and that overall, lake N(2)O production and emission are not raised as significantly as expected due to high N inputs. In comparison, the heavily eutrophied river network is an important fraction of the local N(2)O budget, and when considered together with emissions of N(2)O from the lake, constitute a major (10-50% depending on season) fraction of total N(2)O emissions from the Lake Taihu drainage basin.

Relationships between persistent organic pollutants and carbonaceous materials in aquatic sediments of Taiwan

Recent studies have shown that many persistent organic pollutants (POPs, e.g., polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and various pesticides), are strongly associated with carbonaceous materials (including organic carbon (OC) and black carbon (BC)). We hypothesize that carbonaceous materials can be used as a first-order pollution index, to indicate areas where POP pollution may require further investigation. We tested our hypothesis and found that strong, positive correlations between BC and OC contents versus the concentrations of PCBs (and PAHs) existed in estuarine sediments of the Danshui River in 2005 and 2008. Thus, our preliminary results demonstrate that POC and BC are potential indicators of the POP pollution potential in fluvial sediments of the Danshui River in Taiwan. This innovative approach can provide a simple, relatively inexpensive and expedient means to monitor concentrations of POPs in polluted aquatic sediments of Taiwan, and/or those having a legacy of POP inputs.

Invariance of isotope ratios of lithogenic radionuclides: more evidence for their use as sediment source tracers

Activities of radionuclides in the 238U (230Th, 226Ra, 210Pb) and 232Th (232Th, 228Th, 228Ra) decay series were determined in sediments from an east Texas watershed and examined with isotope ratios and compared to particulate organic carbon (POC), % fines (<63 microm) and total concentrations of Al, Fe and Mn. The objective was to elucidate the presence or absence of relationships affecting the effectiveness of these radionuclides in modeling sediment transport. Strong positive correlations were observed between radionuclides and Mn (Th) and % fines (Ra and Th). Isotope ratios effectively reduce these influences, supporting the contention that isotope ratios offset extrinsic variability in terrestrial sediments. Strong associations of 210Pbxs (excess 210Pb) and 226Ra/230Th with POC agree with data from marine and terrestrial settings, indicating that the role of POC in isotope fractionation, transport and sequestration merits further investigation.

Artificial modifications of the coast in response to the Deepwater Horizon oil spill: Quick solutions or long-term liabilities?

The Deepwater Horizon oil spill threatened many coastal ecosystems in the Gulf of Mexico during the spring and summer of 2010. Mitigation strategies included the construction of barrier sand berms, the restriction or blocking of inlets, and the diversion of freshwater from rivers to the coastal marshes and into the ocean, in order to flush away the oil, on the premise that these measures could reduce the quantity of oil reaching sensitive coastal environments such as wetlands or estuaries. These projects result in changes to the ecosystems that they were intended to protect. Long-term effects include alterations of the hydrological and ecological characteristics of estuaries, changes in sediment transport along the coastal barrier islands, the loss of sand resources, and adverse impacts to benthic and pelagic organisms. Although there are no easy solutions for minimizing the impacts of the Deepwater Horizon disaster on coastal ecosystems, we recommend that federal, state, and local agencies return to the ...

Sources of alluvium in a coastal plain stream based on radionuclide signatures from the 238U and 232Th decay series

ratios 226 Ra/ 232 Th, 226 Ra/ 230 Th, and 230 Th/ 232 Th. Channel alluvium indicates a transition from interfluve surface to subsoil sources during flood (subsoil � 34% to � 91%, over about 8 km) and bank-full stages (subsoil � 9% to � 74%, over about 12 km), with distance downstream. These results indicate strong coupling between hillslope and channel processes, reflecting land use change from forested to agricultural, concentrated in lower Loco Bayou. This methodology shows that sediment sources can be differentiated based upon landscape placement where lithologic contrast is absent. The geochemistry, long halflives, and fractionation of 238 U and 232 Th decay series radionuclides during pedogenic and fluvial processes in humid climates suggest that these methods are applicable in a wide variety of fluvial systems. INDEX TERMS: 1040 Geochemistry: Isotopic composition/chemistry; 1803 Hydrology: Anthropogenic effects; 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); KEYWORDS: natural radionuclides, fingerprinting, alluvium, source apportioning

A 60-year sedimentary record of natural and anthropogenic impacts on Lake Chenghai, China.

Recent sediments from Lake Chenghai, China, were investigated at high temporal resolution to trace both natural and anthropogenic effects on the lake using total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), organic phosphorus (Po), inorganic phosphorus (Pi) and organic carbon and nitrogen stable isotopes (delta13Corg and delta15N) in a 137Cs-dated sediment core. The results indicated that the sedimentary record covers the last 60 years, during which the lake had undergone apparent changes in nutrient sources and productivity in response to nutrient loading. Prior to the late 1980s, the nutrient contents in sediments mainly originated from algae and lake productivity was relatively stable. Since the late 1980s, increasing TOC, TN and TP concentrations together with the change of delta13Corg and delta15N suggested anthropogenic perturbations in nutrient loading and lake productivity. Endogenic nutrients derived from algae and anthropogenic inputs were two important sources of sedimentary nutrients. The anthropogenic nutrients mainly originated from the discharge of industrial wastewater and artificial cultivation of Spirulina after the middle 1980s, and domestic wastewater discharged from Yongsheng County since 1993.

Impacts of Dredging Activities on the Accumulation of Dioxins in Surface Sediments of the Houston Ship Channel, Texas

Abstract The Houston Ship Channel (HSC) and upper Galveston Bay (GB), Texas, are known to be contaminated with dioxins (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans), the majority of which are associated with sediments. Since 1914, dredging operations to establish, sporadically expand, and consistently maintain a navigable channel for large ships has been and remains continuous here. The objectives of this research focus on determining if dredging activities have any significant impact on the quantities of dioxins associated with surface sediments in the HSC and GB. Four transects were sampled, located on the dredged and undredged sides of two dredge-spoil islands. Sediment samples were characterized in terms of their organic carbon contents, grain size fractions, indicator dioxin concentrations (2,3,7,8-tetrachlorinated dibenzo-p-dioxin [TCDD], 2,3,7,8-tetrachlorodibenzofuran [TCDF], and toxic equivalents), and fallout radionuclide activities. The physical and geochemical data were examined using a combination of principal components analysis and one-way analysis of variance. Results of the statistical tests show that (1) sedimentary dioxin concentrations are significantly higher adjacent to the northernmost dredge-spoil island, which is located closer to a recently identified dioxin point source (San Jacinto waste pits); and (2) while mean sedimentary dioxin concentrations were slightly higher for undredged as compared to dredged transect samples as a whole, these differences were not significant (p > 0.05). Mean fallout isotope ratio values (7Be/137Cs, 7Be/210Pbxs) were greater for the dredged sample population, indicating that dredged sediments contain more of the shorter-lived radionuclides (i.e., 7Be), as their buildup is more rapid after dredging than those isotopes with longer half-lives (210Pb) or those for which a longer time has passed since introduction into the environment (137Cs).

The Relative Importance of Terrestrial Versus Marine Sediment Sources to the Nueces-Corpus Christi Estuary, Texas: An Isotopic Approach

Determining sources of sediment to coastal systems is an important and complex problem that figures prominently in a myriad of geological, geomorphological, geochemical, and biological processes. Lithogenic (226Ra,228Ra,228Th,230Th,232Th) and fallout (137Cs,210Pb) isotopes were employed in conjunction with sedimentological methods to determine rates of sedimentation in the Nueces Delta and Nueces-Corpus Christi Estuary and to assess the relative importance of marine versus terrestrial sediment sources to the estuary. Similarity of lithogenic isotope ratios in surface sediments throughout the system precluded a numeric approach to discerning the importance of each of the two large scale sediment sources (terrestrial and marine). A stepwise, graphical examination of discrete lithogenic isotope activity concentrations shows more promise. Terrestrial, marine, and bay sediment means for226Ra versus232Th,226Ra versus230Th, and228Ra versus232Th show that terrestrial and marine sediment sources have different signatures, despite having similar grain size distributions (sands), and that sediment deposited in Nueces and Corpus Christi Bays are indistinguishable from the terrestrial component. Supporting evidence is provided by thorium isotopes,230Th versus232Th,228Th versus232Th, and228Th versus230Th. Nueces Delta sedimentation (0.09–0.53 g cm−2 yr−1) shows a subtle gradient, with rates generally lower in the west and progressively higher moving east, likely reflecting contrasts in land use and topography. Nueces Bay cores differ from those in Corpus Christi Bay in that sands comprise a larger percentage of their composition, and they are mixed over greater depth, most likely due to geographic and physiographic effects. Sediment accumulation rates consistently decrease over the first 20 km from the Nueces River and become constant after that, implying that the river is the most significant source of sediment to the estuary. The interpretation of sediment supply to this estuary as dominated by terrestrial inputs is based on three complimentary sets of data: sediment grain size distributions, discrete lithogenic isotope data (Ra versus Th and Th versus Th), and sediment accumulation rates for both Nueces and Corpus Christi Bays.

On the academic job hunt, is there not a better way?

After a decade or more of higher education, postdoctoral experience, the papers, proposals, rejections, and successes, one might think that earning a post as an assistant professor would not be so tough. Wrong. Many consider academia the pinnacle of the job market for Ph.D.s, particularly in the sciences. If we are not the best that we can be, we will not be funded, publish in well-read journals, or land that sought after job in the ivory tower, if that is our desire. Unfortunately the academic job hunt, particularly in the United States, is a labor-intensive, time-consuming, feedback-less, and drawn-out affair where excellence in ones chosen field and likely successful fulfillment of the job requirements are often not the determining factors as to whether a candidate is made the offer.