RELATIONSHIPS BETWEEN THE GEOCHEMISTRY OF PACIFIC FERROMANGANESE CRUSTS AND SEAWATER UNDER MODERN AND ANCIENT OCEAN CONDITIONS

Author

• Coralie Rodriguez, University of Rhode IslandFollow

Date of Award

2026

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Geological Oceanography

Department

Oceanography

First Advisor

Katherine Kelley

Abstract

Ferromanganese (Fe-Mn) crusts are deep-sea chemical sedimentary minerals, precipitating from the ambient seawater atop hard substrate, such as seamounts, ridges, and plateaus throughout Earth’s ocean basins. Amalgamations of positively charged Fe-oxyhydroxide and negatively charged Mn-oxides in these crusts sequester oppositely charged ions and complexes from seawater extending over long temporal scales (average growth rate: 1-10mm/Ma). The properties of Fe-Mn crusts and mechanisms of their formation make them particularly good at concentrating economically valuable metals (e.g., Co, Mn, Fe, Ni, Cu, etc.) as well as recording ocean paleo-chemistry at the time of their formation. Some processes promote Fe-Mn crust formation (e.g., low sedimentation), yet we lack robust constraints on the processes driving enrichments of certain metals within these rocks. Moreover, since Fe-Mn crusts form as layered deposits from seawater, they are important records of ocean chemistry changes through time, which affect the trace element and isotopic compositions of each layer. This dissertation investigates the geochemistry of Fe-Mn crusts and how they record their environment using a suite of novel analytical techniques. In the first chapter, I use Fe-Mn crust samples collected on three E/V Nautilus cruises to three different areas within the Central Pacific Ocean. Samples were collected with the remotely operated vehicle (ROV) Hercules, on which sensors are mounted to track seawater variables (i.e., temperature, oxygen, salinity, depth) at the time of rock collection. I sampled the outer surface of the crust using the top scape method, removing only the upper < 1mm of crust for analysis by solution inductively coupled plasma mass spectrometry (ICP-MS). Elements of interest in Fe-Mn crust samples are compared with seawater variables to resolve seawater-rock relationships. I found that the geochemistry of surface scrapes of Fe-Mn crusts varies locally with seawater oxygen concentration and depth, and larger regional variability within our dataset is influenced by different water masses. In the second chapter, I collected Fe-Mn crust and seawater samples with ROV Hercules on E/V Nautilus to the Chautauqua Seamounts. To better compare modern seawater chemistry to Fe-Mn crust chemistry, I utilized a variety of analytical techniques to determine what constitutes “modern” Fe-Mn crust composition. Here, I present Fe-Mn crust metal abundance data from solution ICP-MS analyses of top scrapes, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of the upper 15 µm, and Electron Probe Microanalysis (EPMA) of the outer 5 µm of the rock samples, as well as dissolved metal concentrations from the seawater samples using ICP-MS. I found that metal concentrations obtained through microanalytical analyses (i.e., LA-ICP-MS and EPMA) of Fe-Mn crust samples exhibit distinct chemical signatures compared with those of the solution ICP-MS analyses of the surface scrapes, likely as a result of difference in spatial scaling of these three techniques and geochemical heterogeneity within samples. I examined the relationship between dissolved metals in the paired seawater samples and metal concentrations in the rock to assess mechanisms of Fe-Mn crust formation. I found no relationship between the dissolved trace element concentrations in the seawater and the rocks. However, using trace element data from a GEOTRACES cruise near our study site, I identified a relationship between Fe-Mn crust Co/Mn ratio and the Co/Mn ratios of particulates from the upper 1500m of the water column, suggesting Co accumulation in Fe-Mn crusts is more representative of particle scavenging processes than dissolved Co and Mn in the seawater. In the third and final chapter, I asses the paleoceanographic reconstruction potential of Fe-Mn crusts through triple oxygen isotope analyses (measurements of d¹⁸O and d’¹⁷O). Previous work demonstrated that the Mn oxide portion of Fe-Mn crusts records and preserves dissolved oxygen isotopic signatures during precipitation. Trends of paired D’¹⁷O-d¹⁸O values of dissolved oxygen (where D’¹⁷O values are calculated as deviations from the terrestrial fractionation line), allow for additional insights into variations in global primary productivity throughout time. No study has measured the triple oxygen isotopic composition down the growth axis of a Fe-Mn crust sample with age constraints. Here, I present high precision measurements of d¹⁸O_MnOx and D'¹⁷O_MnOx in layers of a previously age-dated Fe-Mn crust sample, D11-1 - collected on an R/V Farnella cruise to the Pacific Ocean - that records the past~75Myr, allowing for evaluation of changes in global primary productivity during this time. I found that modern D'¹⁷O_MnOx values are at a minimum, relative to the past 75 Myr, suggesting that modern primary productivity is at its lowest point over that time period. Together, my research advances our understanding of the environments in which Fe-Mn crusts grow by evaluating their geochemical characteristics.

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

Recommended Citation

Rodriguez, Coralie, "RELATIONSHIPS BETWEEN THE GEOCHEMISTRY OF PACIFIC FERROMANGANESE CRUSTS AND SEAWATER UNDER MODERN AND ANCIENT OCEAN CONDITIONS" (2026). Open Access Dissertations. Paper 4602.
https://digitalcommons.uri.edu/oa_diss/4602