Perspectives on heterococcolith geochemical proxies based on high-resolution X-ray fluorescence mapping

Area of Science:

• Paleoceanography
• Geochemistry
• Micropaleontology

Background:

• Heterococcoliths, abundant microfossils, offer potential for geochemical studies but remain underutilized.
• Understanding elemental distribution in heterococcoliths is key to developing new paleoceanographic proxies.

Purpose of the Study:

• To map elemental composition in Middle Jurassic heterococcoliths using high-resolution X-ray fluorescence (XRF).
• To investigate the potential of heterococcoliths as proxies for palaeoceanography, specifically salinity and biomineralization processes.

Main Methods:

• Utilized X-ray fluorescence (XRF) with a 100-nm resolution beam at the European Synchrotron Radiation Facility.
• Analyzed two Middle Jurassic Watznaueria britannica heterococcoliths from Cabo Mondego, Portugal, for elements Sr down to S.

Main Results:

• Identified distinct distributions for Ca, Sr, Mn, Cl, Br, S, and trace metals (K, Fe, Cu, Zn, Rb).
• Observed that clay contamination and diagenesis influenced distributions of K, Fe, Cu, Zn, Rb, V, and Cr.
• Found evidence suggesting chlorine (Cl) and bromine (Br) are within interstitial nano-domains, with Cl potentially serving as a salinity proxy.
• Indicated sulfur (S) incorporation via sulphated polysaccharides, active for at least 170 million years.

Conclusions:

• Heterococcolith elemental mapping reveals insights into biomineralization and diagenetic processes.
• Chlorine distribution in heterococcoliths shows potential as a novel proxy for reconstructing past ocean salinity.
• The study confirms the role of sulphated polysaccharides in heterococcolith formation over geological timescales.