Sediment color cycles and their causes in the western Arctic Ocean and Bering Sea

Abstract

A challenge that limits high-latitude paleo-environment investigations is the lack of foraminiferal shell-bearing sediments for stratigraphy constructions. In the Arctic, the Mn-rich brown layers have been proposed as a new approach to establish glacial-interglacial timescale stratigraphy, but their formation mechanisms remain unclear. Until now, only a limited number of sediment cores with Mn-rich brown layers have been studied, and it has been argued that these layers were formed by diagenesis. During past several years, China expeditions have collected many sediment cores from the western Arctic Ocean and Bering Sea offering an opportunity to investigate sediment color cycles in these regions. In this study, we have studied 14 sediment cores from wide geographic and topographic regions including basin, continental slope, continental shelf, and off-shore deep-sea. First, core images were taken by a line scan camera and high-resolution color reflectance was measured by a spectrometer. Second, core sediments were scanned at 1 cm resolution by an XRF core scanner to obtain element content variations. Third, sediment layers with maxima and minima Mn based on XRF data were analyzed by ICP-OES and ICP-MS to crosscheck the reliability of XRF results and provide precise element concentration. Our results show that 9 cores from slope and basin of the western Arctic bear color cycles which can be effectively indicated by a */ b * ratios, as a * is chromatic value from green to red while b * is chromatic value from blue to yellow. In each of these cores, a */ b * ratios co-vary with contents of Mn and some other redox sensitive trace elements such as Cu, Co, Ni, and especially Mo. Data comparison shows that MnO and Mo concentrations at peak values in the cores gradually decrease, respectively, from 5.22% and 80 ppm on the slope to nearly 1% and 30 ppm in the central basin, whereas minimal MnO and Mo concentrations in these cores always stay around 0.1% and <10 ppm, respectively. By contrast, the 5 sediment cores from Bering Sea, continental shelf and off-shore deep-sea of the Western Arctic show (i) no cycle in color, a */ b *, Mn, or Mo, (ii) no correlation between a */ b *, Mn, or Mo, and (iii) generally very low MnO and Mo contents comparable to average values of weathered continents. Based on independent stratigraphy of several cores and considering previous work in this region, we propose the following mechanism for the formation of sediment color cycles in the Western Arctic. During interglacial, a substantial amount of weathered terrigenous Mn-bearing minerals and Mn2+ containing waters are transported by rivers into continental shelves of the Arctic Ocean, where many minerals are subsequently weathered to release additional Mn2+. Although upper waters on continental shelves are oxidizing, bottom waters are more reducing due to the respiration of organic matter, which prevents Mn2+ in waters from being oxidized into MnO x deposits in sediments. This leaves significant amount of Mn2+ to be transported to the Arctic basin by Pacific waters entered through the Bering Strait. Furthermore, Mn-bearing debris carried by sea ice during winter is moved towards the Arctic basin during summer. On the Arctic slope, Mn2+ is oxidized and deposited in the form of MnO x due to little organic matter, making sediments brown color. Also, most Mn-bearing debris is deposited because summer sea ice boundary is located around the slope. Remaining Mn2+ is then further dispersed into other areas in oxidizing waters by ocean currents and is eventually deposited to form brown sediments at depth. While during glacial periods, cold climate and reduced continent exposure due to more extensive ice cover would decrease Mn transportation to the Arctic, resulting in the formation of yellow or dark-grey sediments with low-Mn. If diagenetic influences can be taken into account, cycles of color and Mn content could be used as indicators for glacial-interglacial cycles in the Western Arctic.