Kristen M. Thyng

Dissolved iron and iron isotopes in the southeastern Pacific Ocean

The Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ56Fe) from three of these stations across a near-zonal transect from 20 to 27°S. We found elevated dFe concentrations associated with the oxygen-deficient zone (ODZ), with light δ56Fe implicating porewater fluxes of reduced Fe. However, temporal dFe variability and rapid δ56Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short-depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 µmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ56Fe data in the deep sea showed evidence for a −0.2‰ Antarctic Intermediate Water end-member and a heavy δ56Fe of +0.55‰ for distally transported hydrothermal dissolved Fe from the East Pacific Rise. These heavy δ56Fe values were contrasted with the near-crustal δ56Fe recorded in the hydrothermal plume reaching Station ALOHA in the North Pacific. The heavy hydrothermal δ56Fe precludes a nanopyrite composition of hydrothermal dFe and instead suggests the presence of oxides or, more likely, binding of hydrothermal dFe by organic ligands in the distal plume.

Origins of Karenia brevis harmful algal blooms along the Texas coast

The dinoflagellate Karenia brevis is the major harmful algal bloom (HAB) species in the Gulf of Mexico. Given that the rapid appearance of K. brevis cannot be explained by plankton growth alone, advection is likely important in bloom initiation. Forward- and backward-moving numerical surface drifters were employed in a numerical model of the Texas–Louisiana shelf to help determine the basic physical mechanisms explaining sporadic interannual occurrences of K. brevis along the Texas coastline. Results from data analysis from the area show that HAB events occur in years in which there are weaker mean downcoast, along-shore wind speeds. The drifter experiments suggest that southern waters play a role in HAB event initiation, providing an offshore source of cells at the end of summer. As winds switch from upcoast to downcoast in early fall, offshore populations of K. brevis are swept southward by wind-driven currents in years with strong downcoast winds. However, when the downshore wind is weak, shoreward Ekman transport creates a convergent flow near the coast that allows cells to concentrate and initiate a bloom.

Texas and Louisiana coastal vulnerability and shelf connectivity

A numerical study of connectivity between the continental shelf and coast in the northwestern Gulf of Mexico using a circulation model and surface-limited numerical drifters shows that despite seasonal changes in winds, the overall connectivity of the shelf with the coastline is similar in the winter and summer, though it extends more offshore in Texas in summer. However, there is a spatial pattern to the connectivity: more of the inner shelf is connected with the coast in Texas as compared with Louisiana. Subsets of the coast do have seasonal variability: the coast near both Galveston and Port Aransas has more connectivity from upcoast in the winter and from offshore and downcoast in the summer. In both seasons, we find drifters reach the Port Aransas coast most frequently, with a stronger trend in the summer. These results are important for assessing likely pathways for spilled oil and other potentially hazardous material.