Rapid lateral particle transport in the Argentine Basin: Molecular 14C and 230Thxs evidence

Abstract

Recent studies have revealed that lateral transport and focusing of particles strongly influences the depositional patterns of organic matter in marine sediments. Transport can occur in the water column prior to initial deposition or following sediment re-suspension. In both cases, fine-grained particles and organic-rich aggregates are more susceptible to lateral transport than coarse-grained particles (e.g. foraminiferal tests) because of the slower sinking velocities of the former. This may lead to spatial and, in the case of redistribution of resuspended sediments, temporal decoupling of organic matter from coarser sediment constituents. Prior studies from the Argentine Basin have yielded evidence that suspended particles are displaced significant distances (100 -1000 km) northward and downslope by strong surface and/or bottom currents. These transport processes result in anomalously cold alkenone-derived sea surface temperature (SST) estimates (up to 6°C colder than measured SST) and in the presence of frustules of Antarctic diatom species in surface sediments from Thxs-based focusing factors of 1.4 to 3.2 at sites where alkenone-based SST estimates were 4 -6 °C colder than measured values. In contrast, alkenone radiocarbon data suggest coeval deposition of marine biomarkers and planktic foraminifera, as alkenones in core-tops were younger than, or similar in age to, foraminifera. We therefore infer that the transport processes leading to the lateral displacement of these sediment components are rapid, and hence probably occur in the upper water column (< 1500 m).

Dinosterols or dinocysts to estimate dinoflagellate contributions to marine sedimentary organic matter?

“…While some of the scatter in the data (Fig. 5c) could originate from the measurement uncertainties affecting both methods, we speculate that it is mostly stemming from hydrodynamic effects that lead to focusing of particles of contrasting densities (Mollenhauer et al 2006). As dinoflagellate cells and dinocysts are both composed only of organic materials of very similar densities, little differential sorting of sterols and dinocysts through lateral transport is probably occurring in the water column (both likely have very similar sinking rates).…”

“…Dinocysts on the other hand likely remain free particles of lower density, and their high molecular weight, highly crosslinked, aliphatic matrix makes them highly resistant to degradation, particularly in anoxic environment (Kokinos et al 1998;Gélinas et al 2001;Arzayus and Canuel 2004). Resuspension of sediments can therefore influence depositional patterns and lead to the spatial and, possibly, temporal decoupling of dinocysts and 4a,23,24-sterols (Mollenhauer et al 2006), particularly in high-energy environments such as ocean margins and slopes, or in the vicinity of river mouths. Only the coupled compoundspecific radiocarbon analysis of 4a,23,24-sterols and dinocysts from individual samples would allow testing this hypothesis.…”

“…Ecological and population dynamics can affect source material because: (1) dinocysts recovered in sediments represent a fraction of the original dinoflagellate populations since only ,15% of species produce fossilizable organic-walled cysts (e.g., Head 1996); (2) while dinosterol is produced almost exclusively by dinoflagellates (Volkman et al 1993), it is not the major constituent of the total sterol pool in all dinoflagellate species (Leblond and Chapman 2002); (3) while dinoflagellates synthesize or contain dinosterol at any stage of their biological cycle, dinocysts are only formed during their dormancy phase (Head 1996); and (4) variations in dinoflagellate populations over time and/or geographical locations, or changes in growth conditions of individual species in the surface ocean, can result in a decoupling between dinocyst and dinosterol inputs to sediments. Postdepositional biogeochemical processes also act as decoupling agents since: (5) cysts from heterotrophic dinoflagellates have been shown to be less resistant to degradation under oxic conditions than cysts from autotrophic species (Zonneveld et al 2001); (6) although the steroidal skeleton is very robust and survives many biogeochemical transformations (Wakeham 1989), sterols are subject to several transformations following deposition to sediments, most notably the oxidation of the alcohol in position C3 to form a ketone, and the reductive hydrogenation of unsaturation(s) present in the B-ring of the sterol backbone and/or side chain, forming stenones, stanones, and stanols; and (7) because the major fraction of sedimentary organic matter, including the sterol pool, is bound to mineral surfaces (Keil et al 1994), whereas intact dinocysts are present as organic particles of low density, hydrodynamic sorting could lead to density-based fractionation of dinosterol and dinocysts (Mollenhauer et al 2006), particularly in high-energy environments.…”

Developments in Radiocarbon Technologies: From the Libby Counter to Compound-Specific AMS Analyses

“…(For the latter, see e.g. Mollenhauer et al 2006). The influx of anthropogenic carbon, mostly from fossil fuel burning and spills have been extensively investigated (Reddy et al 2002a,b;Tanner et al 2004;Slater et al 2005Slater et al , 2006Smittenberg et al 2005;White et al 2005;Wakeham et al 2006).…”

Timescales of lateral sediment transport in the Panama Basin as revealed by radiocarbon ages of alkenones, total organic carbon and foraminifera

“…Mollenhauer et al (2006) showed combined 14 C and 230 Th xs data for the Argentine Basin. In this region, independent evidence exists for lateral displacement of alkenone-bearing particles (Benthien and Müller, 2000;Rühlemann and Butzin, 2006).…”