Ostracod response to habitat change in an early Aptian shelf sea

Abstract

Changes in ostracod assemblage composition from the Lower Aptian of the Lower Saxony Basin (Germany) are investigated to estimate their potential as proxies for palaeoenvironmental habitat changes in an epicontinental setting. The studied palaeoenvironmental factors include oxygen depletion, increasing temperatures during OAE 1a and an increasing water depth caused by the supra-regionally recorded early Aptian transgression. In the late early Aptian, a distinct short-term decrease of water depth is inferred. Five ostracod associations are recognized: two are characterized by a unique composition of species and three are unique by dominance of certain species. The stratigraphic record is unique for one association, all other associations are exclusively occurring in short intervals of the section, but are co-occurring with the sub- and superjacent ostracod association. We observe a change from Veeniacythereis–Acrocythere–Dolocythere-dominated to Cytherella-dominated associations, initiating well before OAE 1a and ending slightly before the upper limit of OAE 1a, most likely triggered by increasing temperatures and affected by increasing water depth. Changing water depth proved to be the most important factor affecting the successive appearance of ostracod associations. Moreover, ostracod associations change in dominance from Cytherella- to Pontocyprella–Schuleridea-and finally to Neocythere-dominated faunas parallel to an increasing water depth. A re-establishment of palaeoenvironmental conditions typical for the Veeniacythereis–Dolocythere-dominated association gave rise to a Saxocythere–Batavocythere-dominated replacement association. Although ostracods are very sensitive organisms with respect to oxygen deficiency, the oxygen depletion during OAE 1a was not a major factor for changing ostracod associations. During black-shale deposition ostracods vanished completely, a recognized change in associations started before the anoxic period and ended afterwards. High resolution ostracod studies appeared to be a very powerful proxy for changing water depth, temperature and oxygenation in early Cretaceous epicontinental settings like the Lower Saxony Basin.