Bound forms, compositional features, major sources and diagenesis of long chain, alkyl mid-chain diols in Lake Baikal sediments over the past 28,000 years

To elucidate the possibility of long chain, alkyl mid-chain diols (hereafter, mid-chain diols) in Lake Baikal sediments over the past 28 ka being paleoproxies, we studied their: (i) bound forms and compositional features, (ii) major sources and (iii) diagenesis during sedimentation. Of the seven bound forms investigated, i.e. free form (free type), three base-hydrolysable types (base-1, base-2, and base-3) and acid-hydrolysable type (acid-1 type) in solvent extractable lipids and base hydrolysable (NB) and acid hydrolysable (NA) types in non-solvent extractable (NONEX) lipids, only three, viz. free, NB, and base-3 types, were found. The relative compositions of the three types were 70-100%, 0-30% and 0-8%, respectively. The most abundant component of the free-type diols was the C32 1,15-diol, whereas that of the base-3 and NB types was the C30 1,13-diol. The average chain length, ratio of total amount of even chain lengths to that of odd chain lengths, the relative composition of positional isomers for each carbon number, and the diol index, obtained using the equation [C30 1,15-diol]/([C30 1,15-diol] + [C32 1,15-diol]) × 100, hardly responded to reported major paleoenvironmental changes around the Lake Baikal region over the last 28 ka. Comparison of mid-chain diol compositions and their bound forms with those in possible sources suggested that the major biological source of all mid-chain diols in the sediments might be the Eustigmatophyceae class of algae. In the alga, the occurrence of mid-chain diols in the five bound forms, free, base-3, acid-1, NB and NA types, was suggested, while the acid-1 and NA types were not found in the sediments. Under the oxic depositional conditions of Lake Baikal, most of the acid-1 and NA type diols and part of the base-3 and NB type diols might be degraded and/or released free-type diols. In the anoxic sediments of the lake, the base-3 and NB diols seem to be resistant to diagenesis. All investigated parameters for the free, base-3 and NB diols in the sediments did not provide any information on paleoenvironments around the lake. On the other hand, among the fluxes of the three diol types, the base-3 type diol flux seems to have responded to the climatic events around the lake, especially over the last 15 ka. Hence, this flux might be a potential paleoproxy for the productivity of the biological source(s) and associated environmental factors.