Testing the Utility of Geochemical Proxies to Reconstruct Holocene Coastal Environments and Relative Sea Level: A Case Study from Hungry Bay, Bermuda

Abstract

On low-lying, tropical and sub-tropical coastlines freshwater marshes may be replaced by salt-tolerant mangroves in response to relative sea-level rise. Pollen analysis of radiocarbon-dated sediment cores showed that such a change occurred in Hungry Bay, Bermuda during the late Holocene. This well-established paleoenvironmental trajectory provides an opportunity to explore if geochemical proxies (bulk-sediment δ 13 C and Rock-Eval pyrolysis) can reconstruct known environmental changes and relative sea level. We characterized surface sediment from depositional environments in Bermuda (freshwater wetlands, saline mangroves, and wrack composed of Sargassum natans macroalgae) using geochemical measurements and demonstrate that a multi-proxy approach can objectively distinguish among these environments. However, application of these techniques to the transgressive sediment succession beneath Hungry Bay suggests that freshwater peat and mangrove peat cannot be reliably distinguished in the sedimentary record, possibly because of post-depositional convergence of geochemical characteristics on decadal to multi-century timescales and/or the relatively small number of modern samples analyzed. Sediment that includes substantial contributions from Sargassum is readily identified by geochemistry, but has a limited spatial extent. Radiocarbon dating indicates that beginning at -700 CE, episodic marine incursions into Hungry Bay (e.g., during storms) carried Sargassum that accumulated as wrack and thickened through repeated depositional events until ~300 CE. It took a further ~550 years for a peat-forming mangrove community to colonize Hungry Bay, which then accumulated sediment rapidly, but likely out of equilibrium with regional relative sea-level rise.

Reliability of mangrove radiocarbon chronologies: A case study from Mahé, Seychelles

“…The δ 13 C values collected as part of the radiocarbon measurements indicate that both bulk and organic concentrate samples are likely sampling a mangrove organic carbon pool (Figure 7), as values are consistent with those measured in other mangrove sediments (Kemp et al, 2019; Khan et al, 2019; McKee et al, 2002; Sen and Bhadury, 2017). Fine root material (of the size 10–63 µm) and/or degraded root material may still be present in relatively large amounts in the organic concentrates at Grand Anse.…”

“…These results suggest organic concentrates isolate an older carbon fraction, which is presumably closer to the ‘true’ depositional age than bulk sediment ages because the larger organic fragments, likely from penetrating roots, have been removed. However, studies on peats from a wide variety of environments worldwide yield vastly different results for organic concentrates versus bulk sediment ages (Brock et al, 2011; Kemp et al, 2019; Kim et al, 2021; Strunk et al, 2020; Thomas et al, 2019; Woodroffe et al, 2015b; Wüst et al, 2008). The range in results demonstrates that, whilst organic concentrates isolate a particular size fraction for dating, they still contain an unknown mix of carbon from an unknown number of sources.…”

Trends in heavy metals, polychlorinated biphenyls and toxicity from sediment cores of the inner River Thames estuary, London, UK

“…Stable carbon isotopes (d 13 C) and C/N ratios have been widely employed to provenance sedimentary organic matter in coasts as well as track lateral and vertical (down core) transformations in organic rich sub-environments such as salt marshes and mangroves. 21,[26][27][28] In this current study, the d 13 29 A downstream transect across the salinity range (9 sites) revealed a step wise change in d 13 C and C/N from POM with signicant riverinephytoplankton to marine values at a salinity 0.4, aer-which these values were maintained out to sea. In contrast POM d 15 N ranged from 7.8 to 11.7 but did not exhibit a clear seaward trend.…”

“…Sediments were treated with 1 M HCl, le overnight, washed with deionized water, and oven dried at 60 C. 20 Analysis was performed with a Europa Scientic Elemental Analyser. 21 The reference material used for d 13 C analysis was A-R001 (d 13 C ¼ À26.43&). For quality control purposes check samples of IA-R001, IA-R005 (d 13 C ¼ À26.03&), and IA-R006 (d 13 C ¼ À11.64&) were also analysed during the batch analysis of the samples.…”

Implications of Single-Step Graphitization for Reconstructing Late Holocene Relative Sea-Level Using Radiocarbon-Dated Organic Coastal Sediment

“…Samples were then oven dried at 40°C. Bulk mangrove sediment from Nanitipw and Pukusruk was sieved with MilliQ water to retain the fraction <500 μm and remove large root fragments that are likely to be anomalously young (Kemp et al 2019;Woodroffe et al 2015b). This step ensured that the retained sediment was well mixed (homogenized).…”