Paleoceanography of the Eocene Central Arctic Basin Based on Geochemical Measurements of Biogenic Matter
Abstract
The IODP Leg 302 Arctic Coring Expedition retrieved approximately 120 m long continuous Eocene section from the Lomonosov Ridge in the central Arctic for the first time. The Eocene section is lithologically classified into Unit 2 with the assigned age of ~49-44Ma below and Unit 1/6 with 44Ma above the unit boundary. Unit 2 consists of very dark gray mud bearing siliceous ooze and Unit 1/6 consists of very dark gray silty clay to clayey silt. Biogenic opal (293 samples), organic carbon, nitrogen, and sulfur (150 samples) were measured. The sample resolutions are approximately 16 kyrs for Unit 2 and 8 kyrs interval for Unit 1/6, respectively. In Unit 2 biogenic opal ranged from 40 to 70% whereas in Unit 1/6, it decreased down to 10-20%, except for a peak of 37% at approximately 203 mbsf. Organic carbon consistently showed high values such as 1.4-5.7% both in Units 2 and 1/6. C/N ratios ranged from 15 to 20. Sulfur values ranged 2.6-7.7% in Unit 2 and it significantly increased to 4.0-19.4% in Unit 1/6. Based on the presence of abundant framboidal pyrites and the observed extremely low C/S ratios of <1.2 clearly indicate that the basin was under euxinic marine condition with sluggish bottom water circulation. Siliceous plankton mainly contributed to the biological productivity from 250 to 220 mbsf in Unit 2 and from 216 to 207 mbsf in Unit 1/6, judging from positive correlations of %biogenic opal vs %organic carbon. In other parts of the section such a positive correlation was not seen, implying something else other than siliceous plankton mainly contributed the total productivity. From Unit 2 to Unit 1/6, three rather drastic environmental changes are suggested: decrease of siliceous plankton productivity, increase of sulfate availability, and intensification of the euxinic conditions. In Unit 1/6 where sulfur values significantly increased, an increase of marine water inflow from the North Atlantic suggested by a study on silicofragellates assemblages. The marine water inflow appears to explain the raised dissolved sulfate concentration, resulting more pyrite formation. The conclusions derived from other microfossil studies are also conformable with the results of this study; they are: the Eocene conditions were interpreted as brackish water, fresh water at the top, marine water lying subsurface, eutrophic, relatively shallow, sluggish bottom water circulation, and occasional pelagic water invasions at least in the upper layer.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.U33A0020T
- Keywords:
-
- 4900 PALEOCEANOGRAPHY (0473;
- 3344);
- 4948 Paleocene/Eocene thermal maximum