Current and past research projects
1. Application of a robust paleothermometer for terrestrial paleoclimate studies
Temperature trends and precipitation patterns shape climate on time scales of all lengths. Various proxies, chemical, physical, and biological, have elucidated these changes in both the marine and terrestrial domains over geologic time but are not immune to error. Oceanography traditionally is the surrogate for climate research both past and present as the oceans cover more than two thirds of Earth’s surface. This final third, land masses, account for a vastly greater portion of biodiversity in comparison. Determining preceding environmental parameters on land is crucial to understanding not only former patterns in evolution and diversity but also applies to estimating the effects of modern human-initiated climate change.
The optimal terrestrial climate proxy is easily accessible temporally and spatially and avoids the issues induced by diagenesis which can alter the recorded climate signal. Vertebrate enamel is an excellent listener, because, as essentially pure biogenic apatite, it is highly resistant to alteration. I focus on the ganoine scales of fish of the archaic Lepisosteidae family, colloquially termed gar.
Using the enamel in gar scales from a latitudinal temperature gradient in the eastern United States, I reassessed the oxygen isotopic relationship between environmental water, δ18Owater, oxygen stored in the phosphate of bioapatite, δ18Ophosphate, and temperature. Because δ18Owater is governed by temperature, an independent temperature proxy is needed so as to isolate its value when measuring δ18Omineral, may it be carbonate or phosphate. To this end, I measured the clumped isotopic composition (Δ47) of gar scales to derive a gar-specific, thermodynamically derived paleothermometer. The significant correlation between these variables is retained from previous calibrations, although it differs from those using marine fish teeth (δ18Ophosphate) and carbonates (Δ47). Recent sequencing of the gar genome shows that their enamel is produced in much the same way as in modern mammals, implying that there may be a vital effect associated with enamel-specific enzymes.
2. Phosphorus cycling and coupling with other nutrients and implications on modern environmental processes
Phosphorus is an essential nutrient that has been applied extensively to agricultural land to improve production. Surplus phosphorus has leeched into watersheds worldwide, negatively impacting water quality. Eutrophication occurs when excessive phosphorus or nitrogen promotes algal blooms, eventually resulting in anoxia. Water quality is also negatively impacted by saltwater intrusion, whereby sea level rise and groundwater depletion in coastal regions impairs aquifers used for drinking water and agriculture. Increasing salinity promotes phosphorus release, further exacerbating seasonal eutrophication.
2.1 Effect of salinity on phosphorus cycling at the mesoscale using the Murderkill River
The Murderkill River is a perverse pronunciation of the Dutch “moeder” or mother, and “kill” or body of water. Bordered by salt marshes, it is a tidally influenced river that drains into Delaware Bay, with its western reaches fed by the Columbia aquifer. The phosphorus variation in the watershed, tidal influence of the river itself, manageable watershed size, minimal urban influence, and large amount of historical water quality data makes the Murderkill River ideal for studying salinity’s effects on the biogeochemical cycling of phosphorus, nitrogen, and carbon. Historically high levels of phosphorus have plagued this river, from agricultural nonpoint sources and the point source of the Kent County Regional Resource Recovery Facility (KCRRRF). Point source phosphorus and nitrogen pollution from the wastewater treatment plant have greatly diminished since 2001 when TMDLs for the watershed were implemented by DNREC. There is still a notable summer phosphorus flux from agricultural sources. Isotopic and elemental fingerprinting results show that farm soil significantly contributes to suspended particulate matter in the river.
I am working with an undergraduate to process seasonal data on the concentration and isotopic composition (δ18Op) of dissolved inorganic phosphate (DIP) and the isotopic composition of particulate matter (δ15N and δ13C). Notably, elevated DIP levels downstream of the KCRRRF are only perceptible in May when agricultural phosphorus influence is minimal.
2.2 Effect of salinity on phosphorus cycling at the microscale using controlled experiments
Soil was taken from a coastal estuary in Rehoboth Bay, DE, and exposed to freshwater, brackish, or saltwater treatments, in small sealed bottles over the course of two weeks. Changes were measured in the concentration of different phosphate (P) pools, bulk nitrogen isotopes (δ15N), bulk carbon isotopes (δ13C), and microbial parameters, including alkaline phosphatase activity (APA), dehydrogenase activity (DHA), and enzyme expression (phoD, phoX, amoA, nifH, and nosZ). Enzyme expression analyses were done in collaboration with Marco Burgos at the Universidad de La Frontera, Temuco, Chile.
A decrease in the biologically available P pool over time shows microbial utilization. Calcium-bound P is released over time and transferred to the iron oxide and aluminum-oxide P pool, with the extent of release increasing with salinity. The supernatant is initially the main carbon source but carbon isotopic data indicate that the brackish and saltwater samples switched carbon sources after a few days. Nitrogen isotopic data confirms ammonium mobilization with salinization. This ammonium in turn is converted to nitrate through enzymatic processes, as confirmed by gene expression data. A non-zero amount of reduced iron (Fe2+) present in the initial soil, likely as green rust, acts as an abiotic reducer of nitrite, also confirmed by δ15N data.
2.3 Role of anoxia in phosphorus cycling in Chesapeake Bay
The Chesapeake Bay suffers from varying degrees of water quality issues fueled by both point and non-point nutrient sources. The limited understanding of phosphorus (P) cycling dynamics in sediments prevents accurate estimate of P efflux from sediment to overlying water and impacts water quality. Multiple policies have been implemented to reduce nutrient runoff primarily from agricultural sources, but the water quality has consistently remained subpar.
The flux of P at the sediment-water interface is primarily driven by P release from organic matter degradation, namely remineralization (C-P), and from the reductive dissolution of P-bound Fe-oxides (Fe-P). Bottom water redox conditions control these P effluxes at the sediment-water interface. Yet P flux under C-P vs Fe-P pathways and stability of sinks are drastically different. I seek to (1) identify the control of bottom-water hypoxia on coupled C-P vs Fe-P pathway for P cycling, (2) quantify P sink in sediments, in particular authigenic apatite-P and vivianite-P, and (3) quantify P efflux from sediment-water interface under different pathways and develop relationships with water quality. Answering these fundamental processes will address longstanding questions on sediment response and P efflux and feedback thereof. This research is instrumental in providing data to the Chesapeake Bay program for the adequate formulation of policies that can have tangible effects on water quality improvement.
2.4 Calcite–phosphorus co-precipitation on aquatic vegetation as a P sink
Submerged aquatic vegetation (SAV) are rooted vascular macrophytes that that grow underneath the water surface. They serve as a food source and habitat to organisms, provide dissolved oxygen, and reduce erosion. Perhaps most importantly, both submerged and emergent aquatic vegetation serves as a nutrient sink, although the extent of nutrient removal is species dependent. Emergent macrophytes obtain nutrients from the sediment while floating and SAV remove nitrogen and phosphorus directly from the water. A second Pi removal mechanism intrinsic to both algae and SAV is Pi sorption to precipitated calcite (CaCO3). Calcification in SAV is a byproduct of specialized photosynthesis, as these fully aquatic plants cannot use atmospheric CO2. They instead use dissolved CO2 or HCO3– as a carbon source.
I collected numerous submerged aquatic vegetation (SAV) samples from the Susquehanna River Flats during the summer as they are known to precipitate calcite nodules under high pH (9 and above). Both observational studies in lakes and experimental studies in the lab have looked at how phosphate coprecipitates with calcite. I am specifically interested in the conversion of this CaCO3-P to hydroxyapatite (HAP) or intermediary calcium-phosphate phases, including amorphous calcium phosphate (ACP) and octacalcium phosphate (OCP), either on the plant leaves or in local suspended particulate matter. I wish to explore 1) observational or experimental methods that prove that either the CaCO3-P to OCP to HAP conversion occurs and 2) biogeochemical modeling of the efficiency of this process as a P sink.
From left to right: Collecting plant samples from the Susquehanna Flats in August 2022. SEM image (5k magnification) of a calcite crystal on Vallisneria americana. Large white nodules on lyngbya algae are assumed to be primarily calcite.