Unusual C35 to C38 alkenones in mid-Holocene sediments from a restricted estuary (Charlotte Harbor, Florida)

Abstract

Unusual C 35 to C 38 alkenones were identified in mid-Holocene (8-3.5 kyr BP) sediments from a restricted estuary in southwest Florida (Charlotte Harbor). The distribution was dominated by a C 36 diunsaturated (ω15,20) ethyl ketone, identical to the one present in Black Sea Unit 2 sediments. Other unusual alkenones were tentatively assigned as a C 35:2 (ω 15,20) methyl ketone, a C 37:2 (ω 17,22) methyl ketone and a C 38:2 (ω 17,22) ethyl ketone. In late Holocene sediments < 3.5 kyr BP, the common C 37 to C 39 alkenones were found. Compound-specific 14 C, 13 C, and D isotope measurements were used to constrain the possible origin of the alkenones. Conventional radiocarbon ages of alkenones and higher plant-derived long chain n-alcohols indicated no significant difference in age between mid-Holocene alkenones and higher plant n-alcohols. Both alcohols and alkenones were offset vs. calibrated ages of shell fragments in the same sediment core, which suggests they were pre-aged by 500-800 yr, implying resuspension and redistribution of the fine-grained sedimentary particles with which they are associated. The hydrogen isotopic (δD) composition (-190‰ to -200‰) of the C 37 and C 38 alkenones in the late Holocene sediments is in line with values for coastal haptophytes in brackish water. However, the unusual C 36 and C 38 alkenones from the mid Holocene sediments were enriched in D (by ca. 100‰) vs. the late Holocene alkenones. Also, δ 13 C values of mid-Holocene alkenones were consistently offset compared with late Holocene alkenones (-21‰ to -22‰ and -22‰ to -23‰, respectively). We suggest that the alkenones in Charlotte Harbor were produced by unknown alkenone-producing haptophyte.

Discovery of alkenones with variable methylene‐interrupted double bonds: implications for the biosynthetic pathway

“…Similarly, alkenones with shorter carbon chain lengths (e.g., C 35 –C 36 ) have been reported in estuary sediments from southwest Florida (van Soelen et al. ), late Pleistocene sediments from the Japan Sea (Fujine et al. ), Unit II sediments from the Black Sea (Xu et al.…”

“…, van Soelen et al. ). Therefore, it is likely that the double bond closest to carbonyl group for C 35:3 Me produced by E. huxleyi CCMP2758 is located either at (i) ∆ 7 , five‐methylene interrupted relative to the carbonyl group but three‐methylene interrupted relative to the ∆ 12 double bond; (ii) ∆ 5 , five‐methylene interrupted relative to the ∆ 12 double bond but three‐methylene interrupted relative to the carbonyl group; or (iii) ∆ 26 , five‐methylene interrupted relative to the ∆ 19 double bond.…”

“…, van Soelen et al. ), raises important questions about the biosynthetic processes responsible for the desaturation of the shorter‐chain alkenones. To explain these unusual three‐methylene‐interrupted alkenones, Rontani et al.…”

“…, van Soelen et al. ). The relative abundance of C 35 and C 36 alkenones is also different between cultured E. huxleyi (CCMP1742) and environmental samples: C 35:2 Me, instead of C 36:2 Me, is the predominant alkenone in E. huxleyi culture samples, whereas C 36:2 Et was the predominant alkenone in all the reported sediment and particulate matter samples (Rontani et al.…”

“…, van Soelen et al. ). These observations, in particular the abrupt shifts in alkenone production observed in the CCMP1742 mutant strain of E. huxleyi under standard culture conditions (Prahl et al.…”