Research

As a paleoceanographer and paleoclimatologist, my goal is to understand how the oceans and climate of the past behaved, and why.

I am particularly interested in ocean oxygenation during past warm periods like the Pliocene (~3 Ma) and Miocene (~16 Ma) climate optima, analogs for our future warmer world. I study how oxygen minimum zones responded to climate change, tectonics, and ocean circulation during those times.

Graph showing global surface temperature change on the y-axis and time on the x-axis for the past 60 million years and into the future. The Miocene climate optimum and Pliocene climate optimum share shown with arrows, and horizontal lines show that their temperatures were in line with future projections for global temperature beyond 2100. This figure is modified from the 2021 IPCC report.

Oceanic oxygen minimum zones of the past

Oxygen minimum zones (OMZs) or "ocean dead zones" occupy large areas of the equatorial oceans. They have expanded in recent decades as the oceans warm, yet future models disagree about their future beyond 2100. I study how the two largest OMZs responded to a past warm period, the Miocene climate optimum ~16 million years ago. Multiple lines of geochemical evidence suggest that both areas were better oxygenated at that time than the cold period that followed, suggesting that recent deoxygenation may ultimately reverse.

Global map showing oxygen concentration at 400 m water depth. The oxygen minimum zones in the tropical Pacific and Arabian Sea/Bay of Bengal pop out as bright red areas with low oxygen concentrations. Image made in Ocean Data View using WOD18 data.

Image: Oxygen concentration at 400 m water depth. Note the oxygen minimum zones in the tropical Pacific and Arabian Sea/Bay of Bengal. Made in Ocean Data View using WOD18 data.

Related publications:

Hess, A.V., Auderset, A. (co-first authors), Rosenthal, Y., Sigman, D., Martínez-García, A., submitted, Limited Oxygen Deficiency in the Arabian Sea during the Miocene.

Hoogakker, B., Davis, C., Wang, Y., Kusch, S., Nilsson-Kerr, K., Hardisty, D., Jacobel, A., Reyes Macaya, D., Glock, N., Ni, S., Sepúlveda, J., Ren, A., Auderset, A., Hess, A.V., Meissner, K., Cardich, J., Anderson, R., Barras, C., Basak, C., Bradbury, H., Brinkmann, I., Castillo, A., Cook, M., Costa, K., Choquel, C., Diz, P., Donnenfield, J., Elling, F., Erdem, Z., Filipsson, H., Garrido, S., Gottschalk, J., Govindankutty Menon, A., Groeneveld, J., Hallman, C., Hendy, I., Hennekam, R., Lu, W., Lynch-Stieglitz, J., Matos, L., Martínez-García, A., Molina, G., Muñoz, P., Moretti, S., Morford, J., Number, S., Radionovskaya, S., Raven, M., Somes, C., Studer, A., Tachikawa, K., Tapia, R., Tetard, M., Vollmer, T., Wu, S., Zhang, Y., Zheng, X.-Y., and Zhou, Y., 2025, Reviews and syntheses: Review of proxies for low-oxygen paleoceanographic reconstructions. EGU Sphere, v. 22, https://doi.org/10.5194/bg-22-863-2025.

Hess, A.V., Auderset, A., Rosenthal, Y., Miller, K.G., Zhou, X., Sigman, D.M., and Martínez-García, A., 2023, A well oxygenated eastern tropical Pacific during the warm Miocene. Nature, v. 619, p. 521-525. https://doi.org/10.1038/s41586-023-06104-6.

Ventilation of the North Pacific

The ocean interior receives its oxygen from sinking of well oxygenated surface waters in the North Atlantic and Southern Ocean. Despite being quite cold, the North Pacific is too fresh and therefore buoyant for surface waters to sink, which contributes to the strength of the tropical Pacific oxygen minimum zone. However, there were times in the past when this was not the case, which would have led to oxygenation of the Pacific oxygen minimum zone. My research investigates the possibility that surface waters subducted during two warm periods of the past, the Pliocene and Miocene climate optima (~3 and 16 Ma) and has implications for the ocean's response to continued warming in the future.

Illustration of the ocean conveyor belt with sinking of surface waters in the North Atlantic. A red arrow is added in the North Pacific, symbolizing possible North Pacific subduction of surface waters during the geologic past. This figure is modified from Shimokawa and Ozawa (2011).

Image: Illustration of the ocean conveyor belt with a red arrow is added in the North Pacific, symbolizing possible North Pacific subduction of surface waters during the geologic past. Modified from Shimokawa and Ozawa (2011).

Related publications:

Hoogakker, B., Davis, C., Wang, Y., Kusch, S., Nilsson-Kerr, K., Hardisty, D., Jacobel, A., Reyes Macaya, D., Glock, N., Ni, S., Sepúlveda, J., Ren, A., Auderset, A., Hess, A.V., Meissner, K., Cardich, J., Anderson, R., Barras, C., Basak, C., Bradbury, H., Brinkmann, I., Castillo, A., Cook, M., Costa, K., Choquel, C., Diz, P., Donnenfield, J., Elling, F., Erdem, Z., Filipsson, H., Garrido, S., Gottschalk, J., Govindankutty Menon, A., Groeneveld, J., Hallman, C., Hendy, I., Hennekam, R., Lu, W., Lynch-Stieglitz, J., Matos, L., Martínez-García, A., Molina, G., Muñoz, P., Moretti, S., Morford, J., Number, S., Radionovskaya, S., Raven, M., Somes, C., Studer, A., Tachikawa, K., Tapia, R., Tetard, M., Vollmer, T., Wu, S., Zhang, Y., Zheng, X.-Y., and Zhou, Y., 2025, Reviews and syntheses: Review of proxies for low-oxygen paleoceanographic reconstructions. EGU Sphere, v. 22, https://doi.org/10.5194/bg-22-863-2025.

Pacific Walker Circulation

The Pacific Walker Circulation (PWC) is the atmospheric circulation cell that traverses the tropical Pacific. Its variability on interannual timescales is associated with El Niño and La Niña events, with catastrophic consequences globally, from flooding and droughts to increased spread of disease to economic impacts at the country level. Our ability to predict changes in PWC strength is vital for mitigating these effects. Since 1980, the PWC has strengthened, but future projections suggest that it will ultimately weaken. My work is to reconstruct changes in the PWC on longer timescales through the geologic past, during the Miocene and over the last millennium.

A cartoon of the Walker Circulation. It is a west-east cross section through the tropical Pacific ocean and atmosphere. The atmospheric cells converge over Indonesia, leading to rainfall. They push surface waters westward across the Pacific, leading to a deeper thermocline, higher sea surface, and increased Indonesian throughflow. The thermocline is deeper in the west and shallower in the east, and deep waters upwell in the east. All of this is shown with arrows, and red shading in the West Pacific indicates the warm pool.

Image: Cartoon of the Pacific Walker Circulation.

Related publications:

Hess, A.V., Wang, S., Ummenhofer, C.C., Rosenthal, Y., and Oppo, D.W., submitted, Pacific Walker Circulation strengthening during the Little Ice Age.

Hess, A.V., Auderset, A., Rosenthal, Y., Miller, K.G., Zhou, X., Sigman, D.M., and Martínez-García, A., 2023, A well oxygenated eastern tropical Pacific during the warm Miocene. Nature, v. 619, p. 521-525. https://doi.org/10.1038/s41586-023-06104-6.

Miocene ocean temperature

I am working to reconstruct Miocene ocean temperatures as part of the MioOcean effort. In conjunction with climate modelers, this proxy-synthesis effort seeks to improve model-data mismatches and our understanding of our broader climate system throughout the Miocene (~23-5 Ma).

Related publications:

Sodian, S., Auderset, A. (co-lead authors), Modestou, S., Holbourn, A.E. (MioOcean Steering Committing), and MioOcean Working Group Members, in prep., MioOcean: database compilation and application of temperature proxies in the Miocene oceans.

Sosdian, S., Kochann, K., Hess, A.V. (working group leads), Bolton, C., Drury, A.J., Judd, E., Kuhnt, W., Lear, C.H., and Minton, P. (working group members), in prep., MioMg: Reconstructing robust ocean temperatures from Miocene foraminiferal Mg/Ca records.

Geochemical method development

My primary geochemical methodology is measuring the trace elemental composition of foraminifera, sand-sized microorganisms that are preserved in deep-sea sediments. Though I measure many trace elements, I have been heavily involved in development of the iodine-to-calcium ratio proxy for ocean oxygen concentrations. At Rutgers, we developed the first methodology to measure iodine-to-calcium ratios alongside other trace elements, which allows us to understand how ocean oxygen concentrations interact with things like ocean temperature and nutrient concentrations. I also improved understanding of how the iodine distribution in the oceans relates to what gets preserved in forams and provided a new quantitative calibration. This study nearly doubled the amount of data used for calibration and provided the first separate calibrations for foram species that live at different water depths. I am also now working with redox trace elements in bulk sediment samples.

Cartoon showing the experimental design testing foraminifera fragments treated different ways before measuring their iodine-to-calcium ratios. It shows a series of vials with arrows showing how they moved through the geochemical methodology. From Zhou, Hess et al. (2022).

Image: Cartoon showing the experimental design testing foraminifera fragments with different geochemical treatments, testing effect on iodine-to-calcium ratios. From Zhou, Hess et al. (2022).

Related publications:

Hess, A.V., Rosenthal., Y., Zhou, X., Bu, K., 2025, The I/Ca paleo-oxygenation proxy in planktonic foraminifera: A multispecies core-top calibration. Geochimica et Cosmochimica Acta, v. 393, p. 182-195. https://doi.org/10.1016/j.gca.2025.01.018.

Hoogakker, B., Davis, C., Wang, Y., Kusch, S., Nilsson-Kerr, K., Hardisty, D., Jacobel, A., Reyes Macaya, D., Glock, N., Ni, S., Sepúlveda, J., Ren, A., Auderset, A., Hess, A.V., Meissner, K., Cardich, J., Anderson, R., Barras, C., Basak, C., Bradbury, H., Brinkmann, I., Castillo, A., Cook, M., Costa, K., Choquel, C., Diz, P., Donnenfield, J., Elling, F., Erdem, Z., Filipsson, H., Garrido, S., Gottschalk, J., Govindankutty Menon, A., Groeneveld, J., Hallman, C., Hendy, I., Hennekam, R., Lu, W., Lynch-Stieglitz, J., Matos, L., Martínez-García, A., Molina, G., Muñoz, P., Moretti, S., Morford, J., Number, S., Radionovskaya, S., Raven, M., Somes, C., Studer, A., Tachikawa, K., Tapia, R., Tetard, M., Vollmer, T., Wu, S., Zhang, Y., Zheng, X.-Y., and Zhou, Y., 2025, Reviews and syntheses: Review of proxies for low-oxygen paleoceanographic reconstructions. EGU Sphere, v. 22, https://doi.org/10.5194/bg-22-863-2025.

Zhou, X., Hess, A.V. (co-first authors), Bu, K., Sagawa, T., and Rosenthal, Y., 2022, Simultaneous determination of I/Ca and other elemental ratios in foraminifera using sector field ICP-MS, Geochemistry, Geophysics, and Geosystems, v. 23, e2022GC010660. https://doi.org/10.1029/2022GC010660.