Weak coupling of fluvial aggradation and paleo-denudation rates during the last glacial cycle, Crete, Greece

Richard F Ott, Dirk Scherler, Karl W Wegmann, Mitch K D'Arcy, Richard J Pope, Susan Ivy-Ochs, Marcus Christl, Christoph Vockenhuber, Tammy Rittenour

The impact of Quaternary climate cycles on denudation rates and fluvial aggradation and incision is debated, especially in non-glaciated regions. Here we present paleo-denudation rates and geochronological constraints on aggradation and incision from the Sfakia and Elafonisi alluvial-fan sequences on Crete, Greece. We report seven optically stimulated luminescence (OSL), ten radiocarbon ages, and eight 10Be and eight 36Cl denudation rates from modern and terrace sediments. For five samples, 10Be and 36Cl were measured on the same sample by measuring 10Be on cherts and 36Cl on calcite. Results indicate relatively steady denudation rates throughout the past 80kyr, but the aggradation and incision history indicate a link with shifts in climate. At the Elafonisi fan, we identify four periods of aggradation coinciding with Marine Isotope Stages (MIS) 2, 4, 5a/b, and likely 6, and three periods of incision coinciding with MIS 1, 3, and likely 5e. At the Sfakia fan, rapid aggradation occurred during MIS 2 and 4, analogous to aggradation periods at the Elafonisi fan system, followed by up to 50 m of incision during MIS 1. Nearby climate and vegetation records show that MIS 2, 4, and 6 were characterized by cold and dry climates with sparse vegetation, whereas forest cover and wet conditions prevailed during MIS 1, 3, and 5. Our data thus suggest that past changes in climate had little effect on landscape-wide denudation rates, but exerted a strong control on the aggradation-incision behaviour of alluvial channels on Crete. During glacial stages, we attribute aggradation to hillslope sediment release promoted by reduced vegetation cover and decreased river sediment transport capacity; conversely, incision occurred during relatively warm and wet stages due to increased river transport capacity. We conclude that, in this landscape, past hydroclimate variations outcompeted changes in sediment supply as the primary driver of alluvial deposition and incision.