Sediment accumulation, rather than mixing, controls the temporal resolution of the sedimentological record

Niklas Hohmann , Jack Middelburg, Emilia B. Jarochowska

Sedimentary particles such as organismal remains carry information on the Earth’s past. As a result of mixing in surface sediments, particles of different ages can be found at the same depth (time-averaging), and particles of identical ages can be found at different stratigraphic positions (stratigraphic disorder). This results in simultaneous stratigraphic and temporal blurring of the recorded signal, thus reducing the resolution of the sedimentological record.
Here, we draw on a set of observations from modern marine sediments in which the three principal properties of the surface mixed layer (sediment accumulation, mixing depth, and bioturbation intensity) have been measured together, allowing us to disentangle their relative contributions to time-averaging and stratigraphic disorder. We find that sediment accumulation has the strongest influence on time-averaging in modern marine environments. Time-averaging increases with water depth, and a sediment layer in deep sea environments may represent more than 10 kyr of time. In contrast, stratigraphic disorder is controlled by mixing depth. Surprisingly, particle mixing due to bioturbation has only a weak effect on stratigraphic disorder and time-averaging, as the majority of modern sediments are already thoroughly mixed.
Our results highlight that age-reversals are a common feature of the sedimentological record when sampling below decimeter scale and that the physical processes of sediment mixing and accumulation rather than analytical errors provide an upper limit on the temporal resolution achievable in Holocene marine records. Time-averaging and stratigraphic disorder are the result of the same mixing process in the surface mixed layer, and high temporal and stratigraphic resolution can only be achieved when sediment accumulation is high. Secular increases in mixing depth could have led to a 10-fold decrease in the temporal resolution of the sedimentological record across the Phanerozoic.