Preservation of benthic foraminifera and reliability of deep-sea temperature records: Importance of sedimentation rates, lithology, and the need to examine test wall structure

Preservation of planktic foraminiferal calcite has received widespread attention in recent years, but the taphonomy of benthic foraminiferal calcite and its influence on the deep-sea palaeotemperature record have gone comparatively unreported. Numerical modeling indicates that the carbonate recrystallization histories of deep-sea sections are dominated by events in their early burial history, meaning that the degree of exchange between sediments and pore fluids during the early postburial phase holds the key to determining the palaeotemperature significance of diagenetic alteration of benthic foraminifera. Postburial sedimentation rate and lithology are likely to be important determinants of the paleoceanographic significance of this sediment pore fluid interaction. Here we report an investigation of the impact of extreme change in sedimentation rate (a prolonged and widespread Upper Cretaceous hiatus in the North Atlantic Ocean) on the preservation and delta O-18 of benthic foraminifera of Middle Cretaceous age (nannofossil zone NC10, uppermost Albian/lowermost Cenomanian, similar to 99 Ma ago) from multiple drill sites. At sites where this hiatus immediately overlies NC10, benthic foraminifera appear to display at least moderate preservation of the whole test. However, on closer inspection, these tests are shown to be extremely poorly preserved internally and yield delta O-18 values substantially higher than those from contemporaneous better preserved benthic foraminifera at sites without an immediately overlying hiatus. These high delta O-18 values are interpreted to indicate alteration close to the seafloor in cooler waters during the Late Cretaceous hiatus. Intersite differences in lithology modulate the diagenetic impact of this extreme change in sedimentation rate. Our results highlight the importance of thorough examination of benthic foraminiferal wall structures and lend support to the view that sedimentation rate and lithology are key factors controlling the paleoceanographic significance of diagenetic alteration of biogenic carbonates.