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Geochemical arguments for identifying second‐order sea‐level changes in hemipelagic carbonate ramp deposits
Author(s) -
Rosales Idoia,
Quesada Santiago,
Robles Sergio
Publication year - 2006
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.2006.00684.x
Subject(s) - geology , total organic carbon , seawater , carbonate , excursion , isotopes of carbon , marine transgression , salinity , transgressive , δ13c , sea level , paleontology , oceanography , geochemistry , mineralogy , stable isotope ratio , sedimentary depositional environment , structural basin , chemistry , environmental chemistry , physics , organic chemistry , quantum mechanics , political science , law
Abstract Second‐order transgressive–regressive (T–R) cycles, previously recognized using sedimentological criteria in Lower Jurassic hemipelagic deposits from northern Spain, are distinguishable based upon bulk‐rock organic geochemistry [total organic carbon (TOC) and hydrogen index (HI)] and the stable carbon isotope compositions from belemnite rostra. There is a coincidence between regressions and decreasing δ 13 C bel , TOC and HI values, and between transgressions and increasing δ 13 C bel , TOC and HI values. The δ 18 O and Mg/Ca records from the belemnite rostra are not always in phase with the T–R cycles. The δ 18 O bel record reveals, however, a prominent excursion towards higher values within the spinatum Zone that correlates, according to our results, with a regression and with negative shifts in Mg/Ca, δ 13 C bel and TOC. On the other hand, an excursion in the δ 18 O bel towards lower values in the serpentinus Zone also correlates with a peak transgression and with positive shifts in Mg/Ca, δ 13 C bel and TOC. These two excursions have been identified in other European regions as geochemical perturbations of the same characteristics. This suggests a link between second‐order relative sea‐level changes and variations in seawater geochemistry that may reflect local and regional palaeoceanographic perturbations in sea‐water temperature, salinity and water circulation during the Early Jurassic. Terra Nova, 18, 233–240, 2006