Submarine turbidity currents are a key mechanism in the transportation of clastic sediments to deep seas. Such currents may initiate with a complex longitudinal flow structure comprising flow pulses (for example, by being sourced from retrogressive sea floor slope failures) or acquire such structure during run‐out (for example, following flow combination downstream of confluences). A key question is how far along channel pathway complex flow structure is preserved within turbidity currents as they run out and thus if flow initiation mechanism and proximity to source may be inferred from the vertical structure of their deposits. To address this question, physical modelling of saline flows has been conducted to investigate the dynamics of single‐pulsed versus multi‐pulsed density driven currents. The data suggest that, under most circumstances, individual pulses within a multi‐pulsed flow must merge. Therefore, initiation signatures will only be preserved in deposits upstream of the merging point and may be distorted approaching it; downstream of the merging point, all initiation signals will be lost. This new understanding of merging phenomenon within multi‐pulsed gravity currents broadens our ability to interpret multi‐pulsed turbidites.

Deposits of submarine turbidity currents, turbidites, commonly exhibit upward‐fining grain size profiles reflecting deposition under waning flow conditions. However, more complex grading patterns such as multiple cycles of inverse‐to‐normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow. Such flows are termed multipulsed turbidity currents, and their deposits pulsed or multipulsed turbidites. Pulsing may arise at flow initiation, or following downstream flow combination. Prior work has shown that individual pulses within multipulsed flows are advected forward and merge, such that complex longitudinal velocity profiles eventually become monotonically varying, although transition length scales in natural settings could not be predicted. Here we detail the first high frequency spatial (vertical, streamwise) and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments. The data support both a process explanation of pulse merging and a phase‐space analysis of transition length scales; in prototype systems, the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites. The scaling analysis is limited to quasi‐horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures (<10 km long) that develop progressively up‐dip and predicts pulse merging after only a few tens of kilometers. The model cannot provide quantitative estimation of merging in down‐slope flows generated by axially extensive (>10 km) sequences of breaches or where pulsing arises from combination at confluences of single‐pulsed flows, such flows may be responsible for the pulsing signatures seen in some distal turbidites, >100 km from source.

Previous studies on dilute, multi-pulsed, subaqueous saline flows have demonstrated that pulses will inevitably advect forwards to merge with the flow front. On the assumption that pulse merging occurs in natural-scale turbidity currents, it was suggested that multi-pulsed turbidites that display vertical cycles of coarsening and fining would transition laterally to single-pulsed, normally graded turbidites beyond the point of pulse merging. In this study, experiments of dilute, single- and multi-pulsed sediment-bearing flows (turbidity currents) are conducted to test the linkages between downstream flow evolution and associated deposit structure. Experimental data confirm that pulse merging occurs in laboratory-scale turbidity currents. However, only a weak correspondence was seen between longitudinal variations in the internal flow dynamics and the vertical structure of deposits; multi-pulsed deposits were documented, but transitioned to single-pulsed deposits before the pulse merging point. This early transition is attributed to rapid sedimentation-related depletion of the coarser-grained suspended fraction in the laboratory setting, whose absence may have prevented the distal development of multi-pulsed deposits; this factor complicates estimation of the transition point in natural-scale turbidite systems.