A persistent subtropical dip in the diversity at deep-sea cold-seeps

The faunas around methane seeps in the deep sea thrive on an in-situ food source and are thus expected to show no latitudinal gradients in their diversity. An analysis of 478 modern and 824 fossil (Cretaceous to Pleistocene) occurrences of seep molluscs shows that this is indeed the case. However, seep molluscs from 200 to 1000 m depth show a dip in diversity between 15 and 35° latitude, a pattern that has persisted at least since the Oligocene. This dip in diversity coincides with the distribution of oxygen minimum zones, suggesting oxygen limitation as a main driver of this pattern. The diversity dip is less severe in the Eocene, potentially due to weaker ocean stratification, and hence less distinct bathymetric gradients of oxygen concentration, before the Eocene-Oligocene climate transition. Given the role of oxygen in the diversity of seep faunas and their distribution in the geologic record, it is suggested that oxygen limitation could be responsible for the scarcity of seep faunas in the early Palaeozoic.