Biogenic Opal and Carbonate Accumulation Rates in the Subantarctic South Atlantic: The Late Neogene of Meteor Rise Site 704

Abstract

A preliminary composite depth section was generated for Site 704 by splicing Holes 704A and 704B together over the interval 0-350 mbsf (0-9 m.y.). High-resolution carbonate and opal data from the cores were correlated with the calcium and silicon signals from the GST logging run in Hole 704B to identify missing and disturbed intervals in the cores. Paleomagnetic and biostratigraphic age boundaries were then transferred to the composite depth records to obtain an age model, and sedimentation rates were calculated by linear interpolation between datums. Algorithms relating measured dry-bulk density to carbonate content and depth were generated to produce predicted values of density for every sample. Accumulation rates of bulk, carbonate, opal, and terrigenous sediment components were then computed to generate a record of sediment deposition on the Meteor Rise that has a resolution of better than 200,000 yr for the period from 8.6 to 1.0 m.y. From 8.6 to 2.5 m.y., bulk-accumulation rates on the Meteor Rise averaged less than 2 g/cm 2 /1000 yr and were dominated by carbonate deposition. The first significant opal deposition (6.0 m.y.) punctuated a brief (less than 0.6 Ma) approach of the Polar Front Zone (PFZ) northward that heralded a period of increasing severity of periodic carbonate dissolution events (terrigenous maxima) that abruptly terminated at 4.8 m.y. (base of the Thvera Subchron), synchronous with the reflooding of the Mediterranean after the Messinian salinity crisis. From 4.8 to 2.5 m.y., carbonate again dominated deposition, and the PFZ was far south except during brief northward excursions bracketing 4.2-3.9, 3.3-2.9, and 2.8-2.7 m.y. At 2.5 m.y., all components of bulk-accumulation rates increased dramatically (up to 15 g/cm 2 /1000 yr), and by 2.4 m.y., a pattern of alternating, high-amplitude carbonate and opal cyclicity marked the initiation of rapid glacial to interglaciál swings in the position of the PFZ, synchronous with the "onset" of major Northern Hemisphere glaciation. Both mass-accumulation rates and the amplitude of the cycles decreased by about 2 m.y., but opal accumulation rates remained high up through the base of the Jaramillo (0.98 m.y.). From 1.9 to 1 m.y., the record is characterized by moderate amplitude fluctuations in carbonate and opal. This record of opal accumulation rates is interpreted as a long-term "Polar Front Indicator" that monitors the advance and retreat of the opal-rich PFZ northward (southward) toward (away from) the Meteor Rise in the subantarctic sector of the South Atlantic Ocean. The timing of PFZ migrations in the subantarctic South Atlantic Ocean is remarkably similar to Pliocene-Pleistocene climate records deduced from benthic oxygen isotope records in the North Atlantic Ocean (

Jurassic through Early Cretaceous Sedimentation History of the Central Equatorial Pacific and of Sites 800 and 801

“…is approximately 4 times faster than opal accumulation rates in the present central equatorial Pacific (Lyle et al, 1988), but is considerably less than the average of 2000 g/cm 2 /m.y. associated with the Neogene Polar Front Zone in the sub-Antarctic (Froelich et al, 1991). The Mesozoic Pacific may have had elevated levels of equatorial biogenic silica accumulation that were compensated by lesser high-latitude deposition.…”

Late Quaternary variability of ocean circulation in the southeastern South Atlantic inferred from the terrigenous sediment record of a drift deposit in the southern Cape Basin (ODP Site 1089)

“…9). Peak values appear in MIS 12 and MIS 10 (9.78 and 9.62 g cm 32 kyr 31 ) as well as in MIS 11 (7.4 g cm 32 kyr 31 (Froelich et al, 1991;Diekmann et al, 1999), the MAR Terr values at the drift Site 1089 are high and apparently point to elevated £uxes and focusing e¡ects of terrigenous matter induced by bottom-current action. In the entire Atlantic sector of the ACC, MAR Terr was usually higher during glacial periods (Diekmann et al, 1999.…”

Polar ocean stratification in a cold climate

“…7 and , respectively, while the biogenic opal percentage record for ODP Site 882 in the inset of Fig. 2 was determined by alkaline extraction of silica 25 , with replication indicating a reproducibility of 3%.…”

“…Filled circles indicate ODP sites where biogenic opal accumulation decreased during the late Pliocene (the sediment cores shown in Fig. 2 are in bold), whereas open circles show locations where opal accumulation increased during roughly the same time interval [7][8][9]25,[27][28][29][30] . The broad distribution of the opal decrease within both the Subarctic North Pacific and the Antarctic argues against lateral sediment transport as the cause of the opal accumulation decrease.…”