LACUSTRINE PALEOHYDROLOGY FROM HYDROGEN ISOTOPES OF N-ALKANES

This study employs compound-specific dD analyses for elucidating climate and hydrological variations recorded in Early Cretaceous lacustrine rocks from rift basins along West Africa. We compared dD values of n-alkanes from the Kissenda Formation (Neocomian) in Gabon (23 samples; dD from –85 to –120‰), the Marnes Noire and Toca Formations (Barremian) in Congo (21; –67 to –98‰), and the Denda Formation (Barremian) in Angola (18; –81 to –123‰).

Samples from all four formations contain immature to marginally mature organic matter (OM). Differences in dD among these formations are unlikely to result from OM maturity variations. In Gabon, where samples span a vertical range of 470 m from a single core, bulk and biomarker maturity parameters show an increase in OM maturity from immature at the top of the section to marginally mature at the bottom. A slight (10 to 15‰) overall D-enrichment and the increasing homogenization of dD values are interpreted as progressive thermal alteration of hydrogen isotopic values. However, persistent and large differences between dD values (e.g., up to 25‰, between C₁₇ and C₂₉) as well as large variations in dD values (up to 40‰, between C₁₇ and C₂₉) in closely spaced samples at the bottom of the core indicate that environmental signals are preserved.

Comparing dD values from four formations reveals the following patterns. Samples from the Marnes Noire and Toca Formations in Congo show enrichment in D in comparison with the Kissenda Formation in Gabon and the Denda Formation in Angola. The Kissenda and Denda Formations, however, are isotopically similar. Enriched isotopic values from Congo suggest more evaporative conditions in comparison with Gabon. Paleogeography and paleoclimate models (which indicate more arid conditions in Angola), however, make it unlikely that the isotopic differences among the samples from Congo and Angola can be explained solely by variations in the balance between precipitation and evaporation. Differences in source-water composition (e.g., a proposed marine influence in Angola), subsequently modified by evaporate/precipitation, were perhaps also important.