Age determinations on 4 sediment cores off Iceland

The north Icelandic shelf is partly distinguished by the Tjörnes Fracture Zone featuring numerous active basins in a mud-dominated shelf environment. Late Glacial and Holocene high-resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area. Data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores. The correlation is extended with lithological logs and with magnetic susceptibility records. A detailed multidisciplinary study (including biostratigraphy and tephrochronology) of a late Holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal. yr. Of special interest is the exact timing of a marked drop in sea-surface temperature in the area, indicated by ice rafting debris concentration, to about 50 years before the Hekla 3 eruption, which occurred at 2980 cal. yr BP. This appears to predate most records of a general cooling event in NW Europe by a couple of centuries. Two different possible age models, one based on 14C dates combined with tephra markers and one based on tephra markers alone, are discussed in context with the problem of different marine reservoir ages of the water masses in the area.

(Table 1) K-Ar age determinations on biotite from coarse sand within DSDP Sample 72-516F-76-4,107-115

Qualitative petrographic study of selected clastic horizons within the Eocene section of Hole 516F has revealed the presence of abundant fine-grained lithic fragments, probably volcanic, along with coarser fragments of quartz and feldspar apparently derived from a nearby plutonic terrain. In detail, poor sorting, presence of graded bedding, and an abundance of clay suggest these are turbidite horizons locally derived from a mixed volcanic/plutonic terrain, possibly with some direct contribution from contemporary volcanic ash falls. A progressive increase in plutonic versus volcanic components with time is, however, more consistent with an erosional origin for most of this material. Unusual euhedral dark biotite is abundant in several of the lower clastic horizons; it is most easily interpreted as microphenocrysts weathered in situ out of alkalic volcanic ash. Biotite separated from Sample 516F-76-4,107-115 cm, has been dated by the K-Ar method at about 46 Ma. Alkaline volcanoes active on the Rio Grande Rise in the middle Eocene would be the most probable source of this ash and would be consistent with other evidence for potassic, alkaline volcanism along the Rio Grande Rise and at the Tristan da Cunha hot spot.

Age determinations of sediment cores from the Baffin Shelf

Cores HU82-034-057 and HU84-035-008, Resolution Basin, SE Baffin Shelf, contain 200 and 450 cm, respectively, of sediment that spans the Younger Dryas chron. In both cores the interval is bracketed by 14C dates on foraminifera or molluscs. These sites were close to the margin of the late Wisconsin (Foxe) ice sheet as it flowed toward the Labrador Sea. Prior to 11 ka, both cores record moderate to high accumulations of foraminifera, relatively high del 18O values in planktonic foraminifera, and low values of detrital carbonate. The diatom and percent opal records imply occasional seasonally open water conditions. During part of the Younger Dryas chron both the diatom and opal analyses imply a shutoff of biogenic silica production, suggesting surface water conditions affected by increased sea ice and/or reduced nutrients. In addition, the Younger Dryas interval is marked by an increase in coarse sand and detrital carbonate, a decrease in total organic carbon and foraminifera, and high rates of sediment accumulation. The inferred environment during the Younger Dryas is ice-proximal. In HU82-034-057, the foraminifera and other data suggest a change in conditions during the middle part of the Younger Dryas chron.

Stable isotope ratios on foraminifera and age determinations from sediments of BP00 and BP01 cruises to the Kara Sea

no abstract available

Age determinations and analyses of organic matter of sediments from the Argentine Basin

The first radiocarbon chronology for sediments of the Argentine basin has been determined using accelerator mass spectrometer (AMS) analyses of 54 total organic carbon samples from four box and two piston cores collected from the downstream and upstream sides of two central Argentine Basin mudwaves. Throughout the Holocene, sediment from the geomorphically defined upstream side of each wave accumulated at rates of 30 to 105 cm/1000 years. Sediments from the downstream side of each wave accumulated at rates of 2 to 10 cm/1000 years in the late and early Holocene, while the mid Holocene is characterized by sedimentation rates less than 1.0 cm/1000 years. During the mid-Holocene, increased aridity reduced chemical weathering and the flow of the rivers draining to the continental shelf, causing a concomitant decrease in fine-grained terrigenous input to the basin as evidenced by decreased sedimentation rates, lower N/C ratios, and depleted delta13Corg values. It is estimated that all of the organic carbon deposited in the central basin during the mid-Holocene was of a marine origin. During the late and early Holocene, however, approximately 35% of the organic carbon deposited was of terrestrial origin. Bottom water flow speeds in the late Holocene were estimated using a lee-wave model and found to average 14 cm/s. This estimate is comparable to 10 cm/s mean and 15-20 cm/s maximum flow speeds measured by current meters deployed within the basin. Flow speeds in the Argentine Basin were 10% higher than today from 8000 to 2000 B.P., and are consistent with a general invigoration of thermohaline circulation that began between 9000 and 8000 B.P. It is proposed that the introduction of warm, salty Indian Ocean water into the northern North Atlantic at 9000 B.P. was the mechanism that provided the excess salt needed to stabilize the North Atlantic Deep Water thermohaline circulation system in its present mode.

Age determinations of rocks from Vestfjella and Ahlmannryggen in Antarctica

The Proterozoic country rock at Ahlmannryggen consists of flat lying basaltic lo andesitic lava flows and sedimentary rocks intruded by dioritic sills (Borgmassivet Intrusives). The suites display a typical platform cover. K-Ar age determinations gave maximum ages of about 1200 Ma on the magmatic rocks. All these suites were intruded bv Proterozoic dikes dated also at about 1200 Ma. Localiy the Proterozoic rocks have a slaty cleavage grading into mylonitic texture which strike parallel to the Jutul Penck graben. Such tectonic structures were dated at 525 Ma using syntectonic white micas. Evidence of the break-up of Gondwana during the Early Jurassic/Triassic is given by dikes at Ahlmannryggen and lava flows, dikes and sills at Vestfjella. At Ahlmannryggen the initial rift phase is documented by the development of the Jutul Penck graben and the intrusion of the 200-250 Ma continental-tholeiitic dikes striking parallel to the graben axis. The lava flows, dikes and sills at Vestfjella represent a later stage of the Gondwana break-up at about 180 Ma that probably reflects the initial stage of the opening of the Weddell Sea.

Radiometric age determinations for basements of ODP Sites 123-765 and 123-766

The results of experiments in 40Ar/39Ar age dating using fresh basement material from Sites 765 and 766 of Leg 123 of the Ocean Drilling Program are inconsistent and cannot be used to constrain the basement age of the Argo Abyssal Plain in the Indian Ocean. However, a celadonite sample, which was precipitated during a low-temperature alteration event that affected the basement at Site 765, yielded a K-Ar age of 155.3 ±3.4 Ma. Celadonites, which have been dated using Rb-Sr methods for basement in the Atlantic Ocean (Staudigel et al., 1981, doi:10.1016/0012-821X(81)90186-2) and by K-Ar methods for the Troodos Ophiolite (Staudigel et al., 1986, doi:10.1130/0091-7613(1986)14<72:AASAOC>2.0.CO;2), and for sediments from the Pacific Ocean (Peterson et al., 1986, doi:10.2973/dsdp.proc.92.132.1986) yield ages that are up to 15 Ma younger than the age for the formation of basement. Thus, the celadonite age is retained as a reliable minimum age for basement at Site 765. This radiometric age is inconsistent with biostratigraphic ages, which indicate a maximum of late Berriasian (approximately 140 Ma) for Site 765, but is consistent with geophysical interpretations of marine magnetic anomalies and with the early north-south seafloor spreading history of the Argo Abyssal Plain region of the Indian Ocean.

Ar and U-Th-Pb age determinations for minerals from ODP Holes of ODP Legs 173 and 210

We report U-Pb and 39Ar-40Ar measurements on plutonic rocks recovered from the Ocean Drilling Program (ODP) Legs 173 and 210. Drilling revealed continental crust (Sites 1067 and 1069) and exhumed mantle (Sites 1070 and 1068) along the Iberia margin and exhumed mantle (Site 1277) on the conjugate Newfoundland margin. Our data record a complex igneous and thermal history related to the transition from rifting to seafloor spreading. The results show that the rift-to-drift transition is marked by a stuttering start of MORB-type magmatic activity. Subsequent to initial alkaline magmatism, localized mid-oceanic ridge basalts (MORB) magmatism was again replaced by basin-wide alkaline events, caused by a low degree of decompression melting due to tectonic delocalization of deformation. Such "off-axis" magmatism might be a common process in (ultra-) slow oceanic spreading systems, where "magmatic" and "tectonic" spreading varies in both space and time.