Geochemistry of basalts from ODP Leg 49, North Atlantic Ocean

The geochemistry of basalts recovered from seven sites in the North Atlantic is described with particular reference to minor elements. Three sites (407, 408, and 409) along the same mantle flow line, transverse to the Reykjanes Ridge at about 63°N, provide information on the composition of basalts erupted over a 34-m.y. interval between 2.3 and 36 m.y. ago. At Site 410, at 45°N, penetration into 10 m.y.-old crust west of the ridge axis permits comparisons with young basalts dredged from the median valley at 45°N. Three sites in the FAMOUS area at about 36°N provided material from very young (1 m.y.) basaltic crust (Site 411), and material to test the geochemical coherence of basalts of different ages (1.5 and 3.5 m.y.) on either side of a fracture zone (Sites 412 and 413). These sites complement earlier data from dredged and drilled sites (Leg 37) in the FAMOUS area. At Site 407, four geochemically distinct basalt units occur, with different normative and rare-earth element (REE) characteristics, and there is a clear correlation with magnetic stratigraphy. Yet there is a remarkable consistency in incompatible element ratios between these units, indicating derivation from an essentially similar mantle source. The basalts from the younger sites, 408 and 409, show a similar range of normative and REE variation, but incompatible element ratios are identical to those at Site 407, indicating that basalts at all three sites were produced from a mantle source which was geochemically relatively uniform. Rare-earth differences between the basalts can be interpreted in terms of variations in the degree and depth of partial melting causing HREE (+Y) retention in the source, although there may be some inter-site differences with respect to REE. A similar picture is presented at 45°N. Apparently a range of tholeiitic, transitional, and alkalic basalts were being erupted 10 m.y. ago, which have almost identical geochemical characteristics to those recently erupted in the median valley at 45°N. Incompatible element ratios are markedly different from those recorded at the Reykjanes Ridge. Basalts recovered from the FAMOUS sites are geochemically similar to previous samples recovered from the FAMOUS area, and their incompatible element ratios are similar, but not identical, to those at 45°N. However, total trace element levels are consistently lower than in 45°N basalts, which might imply smaller degrees of partial melting and/or greater depths of magma generation at 45°N, or higher trace element levels in the mantle source at 45°N. Few of the basalts recovered on Leg 49 have the geochemical characteristics of typical "MORB" (e.g., Nazca Plate, Leg 34). The data strongly support models invoking geochemical inhomogeneity in the source regions of basalts produced at the Mid-Atlantic Ridge. However, the data also introduce an additional time factor into such models and demonstrate the uniformity of the mantle source at a particular ridge sector (over periods in excess of 30 m.y.), while emphasizing the marked differences along the ridge. Mixing models invoking "depleted" and "enriched" mantle sources would seem to be inadequate to account for the observed variations.

Synchroneity of Pliocene planktonic foraminiferal datums in the North Atlantic

Leg 94 of the Deep Sea Drilling Project has provided a unique set of paleomagnetically dated cores, taken along a N-S transect in the North Atlantic. High deposition rates in the sediments, combined with the palaeomagnetic ages, have enabled existing planktonic foraminiferal zonations to be tested and a new zonation for the mid- to high latitudes to be erected. The PL zonation of Berggren (1973, 1977) is shown to be adequate as far north as 41°N, although both the LAD's of Globigerina nepenthes and Globorotalia margaritae occur earlier than in tropical regions. North of 41°N these two species have very diachronous LAD's, even though they are common during their range in the northern sites. The new zonation for the mid to high latitude North Atlantic is based on the FAD of G. margaritae, FAD of G. puncticulata, LAD of G. cf. crassula, LAD of N. atlantica, FAD of G. inflata and FAD of sinistrally coiled encrusted N. pachyderma.

(Table 1) Age determination of North Atlantic sediment cores

Four cores raised from the eastern Norwegian Sea and adjacent Norwegian fjords at sites influenced by Atlantic water have been investigated. Oxygen isotope analyses in benthic and planktonic foraminifera are used as a proxy for the paleotemperature development spanning the last 800 years. The cores have been dated using a combination of 210Pb and radiocarbon dates yielding time resolutions of 2-5 years for the last century and 9-25 years beyond this. The proxy records have been compared with instrumental time series covering the last 100 years in order to validate the oxygen isotope measurements as a proxy for paleotemperature. The comparison shows that the paleotemperature variability derived from the oxygen isotope analyses is generally similar to the amplitudes and trends seen in the instrumental time series. In particular, a cooling around 1905-1925 followed by a warming until 1955 is evident in all proxy records as well as in the instrumental time series. Beyond the last century the proxy records show two periods from ~1225-1450 and ~1650-1905(25) when temperatures were 1.3-1.6°C lower than present separated by a period of temperatures periodically comparable to present. The last 80 years represent the modern warming and appear to be the warmest period of the last 800 years. We find that that the ocean temperature variability is comparable to terrestrial reconstructions from the region implying a strong link in the ocean-atmosphere climate system. This suggests that the climate variability in this region beyond the period covered by instrumental time series was also associated with changes in the thermohaline circulation.

Spatially explicit estimates of stock size, structure and biomass of North Atlantic albacore tuna (Thunnus alalunga) in the North Atlantic for the period 1987-2011, compiled from statistics about ICCAT fishery region L6

The development of the ecosystem approach and models for the management of ocean marine resources requires easy access to standard validated datasets of historical catch data for the main exploited species. They are used to measure the impact of biomass removal by fisheries and to evaluate the models skills, while the use of standard dataset facilitates models inter-comparison. North Atlantic albacore tuna is exploited all year round by longline and in summer and autumn by surface fisheries and fishery statistics compiled by the International Commission for the Conservation of Atlantic Tunas (ICCAT). Catch and effort with geographical coordinates at monthly spatial resolution of 1° or 5° squares were extracted for this species with a careful definition of fisheries and data screening. In total, thirteen fisheries were defined for the period 1956-2010, with fishing gears longline, troll, mid-water trawl and bait fishing. However, the spatialized catch effort data available in ICCAT database represent a fraction of the entire total catch. Length frequencies of catch were also extracted according to the definition of fisheries above for the period 1956-2010 with a quarterly temporal resolution and spatial resolutions varying from 1°x 1° to 10°x 20°. The resolution used to measure the fish also varies with size-bins of 1, 2 or 5 cm (Fork Length). The screening of data allowed detecting inconsistencies with a relatively large number of samples larger than 150 cm while all studies on the growth of albacore suggest that fish rarely grow up over 130 cm. Therefore, a threshold value of 130 cm has been arbitrarily fixed and all length frequency data above this value removed from the original data set.