Preprints
https://doi.org/10.5194/bg-2023-83
https://doi.org/10.5194/bg-2023-83
10 May 2023
 | 10 May 2023
Status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Revisiting the applicability and constraints of molybdenum and uranium-based paleo redox proxies: comparing two contrasting sill fjords

K. Mareike Paul, Martijn Hermans, Sami A. Jokinen, Inda Brinkmann, Helena L. Filipsson, and Tom Jilbert

Abstract. Sedimentary molybdenum (Mo) and uranium (U) enrichments are often used as redox proxies to reconstruct bottom water redox changes. However, these redox proxies may not be equally reliable across a range of coastal settings due to varying depositional environments. Fjords vary greatly in their depositional conditions, due to their unique bathymetry and hydrography, and are highly vulnerable to anthropogenic and climatic pressures. Currently, it is unknown to what extent Mo and U sequestration is affected by variable depositional conditions in fjords. Here, we use pore water and sequential extraction data to investigate Mo and U enrichment pathways in sediments of two sill fjords on the Swedish west coast with contrasting depositional environments and bottom water redox conditions. Our data suggest that sedimentary authigenic Mo and U pools differ between the two fjords. At the ir/regularly dysoxic (oxygen = 0.2–2 mL L−1) Gullmar Fjord, authigenic Mo largely binds to manganese (Mn) oxides and to a lesser extent to iron (Fe) oxides; Mo sulfides do not play a major role due to low sulfate reduction rates, which limits the rate of Mo burial. Authigenic U largely resides in carbonates. At the ir/regularly euxinic (oxygen = 0 mL L−1; total hydrogen sulfide ≥ 0 mL L−1) Koljö Fjord, authigenic Mo is significantly higher due to binding with more refractory organic matter complexes, and Mo-Fe-sulfide (S) phases. Uranium is moderately enriched and largely bound to organic matter. We found no direct evidence for temporal changes in bottom water redox conditions reflected in Mo and U enrichments at either Gullmar or Koljö Fjord. While sulfidic bottom waters favor Mo sequestration at Koljö Fjord, enrichment maxima reflect a combination of depositional conditions rather than short-term low oxygen events. Our data demonstrate that secondary pre- and post-depositional factors control Mo and U sequestration in fjords to such an extent that bottom water redox conditions are either not being systematically recorded or overprinted. This explains the large variability in trace metal enrichments observed in fjords and has implications for applying Mo and U as proxies for environmental redox reconstructions in such systems.

K. Mareike Paul et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-83', Anonymous Referee #1, 12 Jul 2023
    • AC1: 'Reply on RC1', Mareike Paul, 16 Aug 2023
  • RC2: 'Comment on bg-2023-83', Anonymous Referee #2, 19 Jul 2023
    • AC2: 'Reply on RC2', Mareike Paul, 16 Aug 2023

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2023-83', Anonymous Referee #1, 12 Jul 2023
    • AC1: 'Reply on RC1', Mareike Paul, 16 Aug 2023
  • RC2: 'Comment on bg-2023-83', Anonymous Referee #2, 19 Jul 2023
    • AC2: 'Reply on RC2', Mareike Paul, 16 Aug 2023

K. Mareike Paul et al.

K. Mareike Paul et al.

Viewed

Total article views: 713 (including HTML, PDF, and XML)
HTML PDF XML Total Supplement BibTeX EndNote
529 160 24 713 38 8 17
  • HTML: 529
  • PDF: 160
  • XML: 24
  • Total: 713
  • Supplement: 38
  • BibTeX: 8
  • EndNote: 17
Views and downloads (calculated since 10 May 2023)
Cumulative views and downloads (calculated since 10 May 2023)

Viewed (geographical distribution)

Total article views: 695 (including HTML, PDF, and XML) Thereof 695 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 20 Nov 2023
Download
Short summary
The coastal seafloor naturally contains trace metals, which are sensitive to changes in oxygen concentrations in seawater: trace metal contents increase with decreasing oxygen concentrations. Studies have used those trace metals as indicators for declining oxygen concentrations in coastal waters. Here we show that in fjords, this relationship works well on longer (decadal) time scales but poorly on shorter (seasonal) time scales. We attribute this to other processes influencing the trace metals.
Altmetrics