Abstract
ABSTRACTSediments deposited in the Loch of Stenness (Orkney Islands, Scotland) during the Holocene transgression, previously dated to between ~5939–5612 bp, were analysed for molecular fossils – lipids and chlorophyll pigments from primary producers – that complement conventional microfossil and lithological approaches for studying past sea‐level change. While microfossil and lithological studies identified a transgression between 102 and 81 cm core depth, key molecular fossils fluctuate in occurrence and concentration between 118 and 85 cm, suggesting an earlier start to the transgression. Terrestrial lipid concentrations decreased and algal‐derived, short‐chain, n‐alkanoic acid concentrations increased at 118 cm, indicating a disruption of the freshwater lake conditions associated with the early stages of the marine transgression. The lipid and pigment analyses provided information that complements and extends that from microfossil analysis, presenting a more complete record of Holocene sea‐level changes and local vegetation changes in the Loch of Stenness. The isostatic stability of Stenness during the Holocene points towards other factors to explain the transgression, such as regional factors and/or melting of the Antarctic ice sheet (which occurred up to 3 ka).
Highlights
Changes in sea level have been studied extensively to understand their causes and impacts on the environment and human populations (Rollins et al, 1979; Hodgson et al, 2009; Bates et al, 2016; IPCC, 2019)
Molecular fossils can be analysed within milligram to gram quantities of sediment, a much smaller scale than the few grams needed for traditional macro‐ and microfossil analyses (Shennan et al, 2015)
At 118 cm there is a change in the organic matter (OM): algal markers become the dominant components, reflecting a relative reduction in terrestrial deposition and increase in algal OM production, possibly enhanced by increased preservation resulting from water column anoxia (Fig. 6a,b,d)
Summary
Changes in sea level have been studied extensively to understand their causes and impacts on the environment and human populations (Rollins et al, 1979; Hodgson et al, 2009; Bates et al, 2016; IPCC, 2019). Environmental proxies for tracking transgressions based on soil and sediment analysis exploit shifts in the populations of plant macrofossils, insects, pollen and spores, foraminifera, While a major challenge in using macro‐ and microfossil evidence is their extent of preservation and/or limited number of specimens, it is often possible to extract organic geochemical molecular fossils incorporated into the sediment matrix Through their relationship to the biological molecules from which they originate, the molecular fossils can reflect specific OM inputs (Fig. 1) and thereby reveal characteristics of, and changes in, the environment inhabited by the source organisms (Poynter and Eglinton, 1990; Meyers and Ishiwatari, 1993; Castañeda and Schouten, 2011). Robust lithological and morphological studies are key to identifying the areas of undisturbed sediments that are ideal for palaeoenvironmental and molecular fossil analyses, avoiding compromised areas (e.g. where bioturbation or burrowing has occurred)
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