I am a biogeochemist interested in marine particles and especially how rare, but important! trace elements cycle through them. From a chemical perspective, marine particles are a complex mixture of living and non-living phases like organisms, bio-minerals, crust-derived rock minerals in dust and sediments, and larger aggregates, all constantly interacting with each other. Particles are important sources and repositories of elements required for marine organisms, much like soil to land-based plants. Understanding particle distributions, composition, and behavior helps us understand how the oceans work as part of larger Earth systems.
At sea, our lab collects particles using large-volume, size-fractionated filtration (McLane pumps) and smaller-scale bottle filtrations. We also use optical instrumentation to observe particles in the ocean. Back in the lab, we measure the bulk composition of particles using multi-element mass spectrometry, often through digestion of filtered particles using chemical techniques. We also use synchrotron light (x-rays produced at National Laboratories) to examine the composition of individual marine particles as small as single organisms. These measurements are then incorporated into regional-scale and global-scale biogeochemical/ecosystem models, often through work with collaborators.
Key questions my group is interested in:
-How are key trace elements (Al, Cd, Co, Cu, Fe, Mn, Ni, P, Pb, Th, Ti, V, Zn—among others!) associated with organisms and minerals at different oceanographic sites? How can we better discern these associations using various analytical and statistical techniques?
-How do different organisms and ecosystems access (and transform) key biologically required elements? For instance, how is Fe from lithogenic dust transformed into and among organisms and inorganic scavenged phases?
-How do particle assemblages change and age over time as they sink from the surface? What are the relative roles of microorganisms and inorganic processes in these transformations?
-Can we link high-resolution optical techniques and lower-resolution chemical data to improve our ability to “see” particles of different chemistries?
-What can models tell us about the major sensitivities or uncertainties for different particle behaviors?
U.S. National Science Foundation—NSFGEO-NERC: “Collaborative Research: Using Time-series Field Observations to Constrain an Ocean Iron Model”, 2018-09-01 to 2021-08-31, OCE #1829819 (co-PI), $440,903
In this project, we are analyzing the detailed composition of the marine iron cycle over a full annual cycle at a well-constrained oceanographic site: the Bermuda Atlantic Time-Series station in the N. Atlantic. Iron availability is a controlling influence over marine ecosystems for huge swaths of the planet but remains highly uncertain in many ways. Our work will examine how Fe is partitioned into organisms and marine particles of various sizes and compositions over the course of the year. In concert with the data collected by multiple collaborators, this work will be used to improve regional and global iron models.
U.S. National Science Foundation—NSF DBI: “FSML: Acquisition of a Raman Microscope at the Skidaway Institute of Oceanography”, DBI #1937671 (co-PI), $207,500
This project is supporting the acquisition of a microscope capable of examining the chemical composition of particles at high spatial resolution. This instrument will be used to study samples of biological, chemical, and geological interest at Skidaway, multiple education programs, and our collaborator institutions.
Ph.D., 2013, MIT/WHOI Joint Program in Chemical Oceanography
B.A., 2004, Williams College, Biology and Chemistry
Ohnemus, D. C., Torrie, R., & Twining, B. S. (2019). Exposing the distributions and elemental associations of scavenged particulate phases in the ocean using basin‐scale multi‐element data sets. Global Biogeochemical Cycles, 33, 725-748. doi:10.1029/2018GB006145
Tagliabue, A., Bowie, A.R., DeVries, T. et al. The interplay between regeneration and scavenging fluxes drives ocean iron cycling. Nat Communications 10, 4960 (2019) doi:10.1038/s41467-019-12775-5
Ohnemus, D.C., Lam, P.J., Twining, B.S.: Optical observation of particles and responses to particle composition in the GEOTRACES GP16 section, Marine Chemistry, 201: 124-136, 2018. doi:10.1016/j.marchem.2017.09.004
Ohnemus, D. C., Rauschenberg, S. , Cutter, G. A., Fitzsimmons, J. N., Sherrell, R. M. and Twining, B. S. (2017), Elevated trace metal content of prokaryotic communities associated with marine oxygen deficient zones. Limnol. Oceanogr., 62: 3-25. doi:10.1002/lno.10363
Ohnemus, D.C., Rauschenberg, S., Krause, J.W., Brzezinski, M.A., Collier, J.L., Geraci-Yee, S., Baines, S.B., Twining, B.S.: Silicon content of individual cells of Synechococcus from the North Atlantic Ocean. Marine Chemistry (187), 2016. doi:10.1016/j.marchem.2016.10.003
Ohnemus, D.C. and Lam, P.J.: Cycling of lithogenic marine particles in the US GEOTRACES North Atlantic transect. Deep Sea Research Part II, 116: 283-302, 2015. doi:10.1016/j.dsr2.2014.11.019