Coupling temporal and spatial dimensions of global sediment flux through lake and marine sediment records

A major obstacle to our ability to assess future impacts of climate and human activities on the global sediment system is the lack of suitable timescales over which long-term processes and system properties may be observed. A long-term view offers the opportunity to observe pre-impact states, trajectories and the history of responses to forcings. The paper explores the scope of sediment archives in lakes and the continental margin as sources of information to inform about spatio-temporal characteristics of total suspended sediment (TSS) flux across spatial scales ranging from small catchments (< 10(3) km(2)) to regional -continental basins (10(3) - 10(6) km(2)), and over timescales varying from decades to millennia. Numerous published studies, representing many geographical regions, provide estimates for sedimentation rates over full- or part-Holocene timescales, based largely on 14 C and Pb-210 dating. Results from small-medium drainage basins show that, during the Holocene, climate has been largely subordinate to human impact in controlling long-term shifts in sediment loads, though the evidence for short-term climate impacts is also clear. The rise in sediment delivery following major human impact is typically 5-10-fold higher than under undisturbed conditions, but may be higher depending upon the environment and intensity of impact. Sediment-source studies suggest that surface soil rarely dominates the sediment load, and that channel and gully sources often dominate even in small basins. For larger drainage basins, the forcing-response mechanisms are less clear and there is evidence to suggest that values for modem fluxes lie close to long-term averages. A more systematic analysis of the relative magnitude of change in sediment flux in different basin sizes reveals a global trend where relative changes diminish with increasing basin size. The findings suggest that small basins are most responsive to impacts and will show the largest changes in sediment flux. In contrast, large basins with effective storage zones and very slow reaction times lead to far weaker levels of spatio-temporal coupling. One implication is that the most rapid increases in sediment flux to the coast are caused by disturbance in small-medium drainage basins feeding directly to the coast. Overall, lake and marine sediment archives provide a range of useful information, often with high temporal resolution, that is difficult to obtain by other means. Future studies may be able to use these datasets to provide a typological classification of sediment regimes according to different spatio-temporal characteristics of their forcing-response mechanisms. (C) 2003 Elsevier Science B.V. All rights reserved.