Anomalously low alkenone temperatures caused by lateral particle and sediment transport in the Malvinas Current region, western Argentine Basin

We analysed the alkenone unsaturation ratio (U-37(K');) in 87 surface sedim ent samples from the western South Atlantic (5 degrees N-50 degrees S) in o rder to evaluate its applicability as a paleotemperature tool for this part of the ocean. The measured U-37(K') ratios were converted into temperature using the global core-top calibration of Muller et al. (1998) and compared with annual mean atlas sea-surface temperatures (SSTs) of overlying surfac e waters. The results reveal a close correspondence (<1.5 degrees C) betwee n atlas and alkenone temperatures for the Western Tropical Atlantic and the Brazil Current region north of 32 degrees S, but deviating low alkenone te mperatures by - 2 degrees to - 6 degrees C are found in the regions of the Brazil-Malvinas Confluence (35-39 degrees S) and the Malvinas Current (41-4 8 degrees S). From the oceanographic evidence these low U-37(K') values can not be explained by preferential alkenone production below the mixed layer or during the cold season. Higher nutrient availability and algal growth ra tes are also unlikely causes. Instead, our results imply that lateral displ acement of suspended particles and sediments, caused by strong surface and bottom currents, benthic storms, and downslope processes is responsible for the deviating U-37(K') temperatures. In this way, particles and sediments carrying a cold water U-37(K') signal of coastal or southern origin are tra nsported northward and offshore into areas with warmer surface waters. In t he northern Argentine Basin the depth between displaced and unaffected sedi ments appears to coincide with the boundary between the northward Bowing Lo wer Circumpolar Deep Water (LCDW) and the southward flowing North Atlantic Deep Water (NADW) at about 4000 m. (C) 2000 Elsevier Science Ltd. All right s reserved.