Foraminifera Iodine to Calcium Ratios: Approach and Cleaning

Helge Winkelbauer,Kathy Cordova‐Rodriguez,Elliott Hamilton,Nicolaas Glock,Zunli Lu,Babette Hoogakker,Charlotte Dormon,Simon Chenery,Jennifer Scott,Dharma Reyes‐Macaya,Phil Holdship

doi:10.1029/2021gc009811

Abstract

AbstractPlanktic and benthic foraminiferal iodine (I) to calcium (Ca) molar ratios have been proposed as an exciting new proxy to assess subsurface and bottom water oxygenation in the past. Compared to trace metals, the analysis of iodine in foraminiferal calcite is more challenging, as iodine is volatile in acid solution. Here, we compare previous analyses that use tertiary amine with alternative analyses using tetramethylammonium hydroxide (TMAH) and ammonium hydroxide (NH4OH) to stabilize iodine in solution. In addition, we assess the effect of sample size and cleaning on planktic and benthic foraminiferal I/Ca. Our stabilization experiments with TMAH and NH4OH show similar trends as those using tertiary amine, giving relatively low I/Ca ratios for planktic and benthic foraminifera samples from poorly oxygenated waters, and high ratios for well‐oxygenated waters. This suggests that both alternative methods are suitable to stabilize iodine initially dissolved in acid. Samples that contain 5–10 specimens show a wide spread in I/Ca. Samples containing 20 specimens or more show more centered I/Ca values, indicating that a larger sample size is more representative of the average planktic foraminifera community. The impact of cleaning on planktic and benthic foraminifera I/Ca ratios is very similar to Mg/Ca, with the largest effect occurring during the clay removal step. The largest iodine contaminations were recorded at locations characterized by moderate to high organic carbon contents. In those circumstances, we recommend doubling the oxidative cleaning steps (4 instead of 2 repetitions) to ensure that all organic material is removed.

Highlights

Oxygen concentrations [O2] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al, 2017)
Planktic foraminifera reside in the upper ocean between the surface and ∼1,000 m depending on species (Schiebel & Hemleben, 2017) and after reproduction the empty calcite tests may be buried in the underlying deep-sea sediments
We focused on the well-oxygenated Agulhas Ridge in the south Atlantic (G. inflata at Ocean Drilling Program (ODP) Site 1088) and Cocos Ridge in the eastern tropical Pacific off Panama with a well-developed OMZ in its subsurface waters (N. dutertrei from ODP Site 1242)

Summary

Introduction

Oxygen concentrations [O2] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al, 2017). To better understand the longer-term oxygen cycle in the past, on time scales typically exceeding centuries to millennia, we can use proxy reconstructions (Moffitt et al, 2015). Reconstructions of low-oxygen environments are commonly restricted to sediments directly immersed by low oxygen waters at the time of interest. Beyond the continental slopes and shelves immersed by these waters, it is more challenging to assess upper ocean oxygen conditions from sedimentary proxy methods. Planktic foraminifera reside in the upper ocean between the surface and ∼1,000 m depending on species (Schiebel & Hemleben, 2017) and after reproduction the empty calcite tests may be buried in the underlying deep-sea sediments. Studying I/Ca of fossil planktic foraminifera tests facilitates the study of seawater redox conditions in the open ocean environment

Methods

Results

Conclusion