Physical oceanography from the TRACTOR project

Primary Objectives - Describe and quantify the present strength and variability of the circulation and oceanic processes of the Nordic Seas regions using primarily observations of the long term spread of a tracer purposefully released into the Greenland Sea Gyre in 1996. - Improve our understanding of ocean processes critical to the thermaholine circulation in the Nordic Seas regions so as to be able to predict how this region may respond to climate change. - Assess the role of mixing and ageing of water masses on the carbon transport and the role of the thermohaline circulation in carbon storage using water transports and mixing coefficients derived from the tracer distribution. Specific Objectives Perform annual hydrographic, chemical and SF6 tracer surveys into the Nordic regions in order to: - Measure lateral and diapycnal mixing rates in the Greenland Sea Gyre and in the surrounding regions. - Document the depth and rates of convective mixing in the Greenland Sea using the SF6 and the water masses characteristics. - Measure the transit time and transport of water from the Greenland Sea to surrounding seas and outflows. Document processes of water mass transformation and entrainment occurring to water emanating from the central Greenland Sea. - Measure diapycnal mixing rates in the bottom and margins of the Greenland Sea basin using the SF6 signal observed there. Quantify the potential role of bottom boundary-layer mixing in the ventilation of the Greenland Sea Deep Water in absence of deep convection. Monitor the variability of the entrainment of water from the Greenland Sea using time series auto-sampler moorings at strategic positions i.e., sill of the Denmark Strait, Labrador Sea, Jan Mayen fracture zone and Fram Strait. Relate the observed variability of the tracer signal in the outflows to convection events in the Greenland Sea and local wind stress events. Obtain a better description of deepwater overflow and entrainment processes in the Denmark Strait and Faeroe Bank Channel overflows and use these to improve modelling of deepwater overflows. Monitor the tracer invasion into the North Atlantic using opportunistic SF6 measurements from other cruises: we anticipate that a number of oceanographic cruises will take place in the north-east Atlantic and the Labrador Sea. It should be possible to get samples from some cruises for SF6 measurements. Use process models to describe the spread of the tracer to achieve better parameterisation for three-dimensional models. One reason that these are so resistant to prediction is that our best ocean models are as yet some distance from being good enough, to predict climate and climate change.

(Table 2) Organic residue at DSDP Hole 76-534A

The kind, sedimentation rate, and diagenesis of organic particles delivered to the North Atlantic seafloor during the Middle Jurassic-Early Cretaceous were responsible for the presence of carbonaceous sediments in Hole 534A. Organic-rich black clays formed from the rapid supply of organic matter; this organic matter was composed of either abundant, well-preserved, and poorly sorted particles of land plants deposited in clays and silty clays within terrigenous turbiditic sequences (tracheal facies) or abundant amorphous debris (xenomorphic facies) generated through the digestive tracts of marine zooplankton and sedimented as fecal pellets. Evidence for the fecal-pellet origin of xenomorphic debris is illustrated. Black clays were also produced in sediments containing less organic matter as a result of the black color of carbonized particles composing all or most of the residues (micrinitic facies). Slowly sedimented hematitic Aptian clays contain very little carbonized, organic debris that survived diagenetic oxidation. In the red calcareous clay sequence of the Late Jurassic, larger amounts of this oxidized debris turned several clay layers black or blackish red. Carbonized debris also dominates the residues recovered in interbedded black and green Albian clays. Carbonization of organic matter in these sediments either turned them black or provided the diagenetic environment for reduced iron. Carbonized debris is also appreciable in burrow-mottled black-green Kimmeridgian clay. The study of Hole 534A organic matter indicates that during the middle Callovian there was a rapid supply of terrigenous organic matter, followed by a late Callovian episode of rapidly supplied xenomorphic debris deposited as fecal pellets. The Late Jurassic-Berriasian was a time of slower sedimentation of organic matter, primarily of a marine dinoflagellate flora in a poorly preserved xenomorphic facies variously affected by diagenetic oxidation. Several intervals of carbonized tracheal tissue in the Oxfordian and Kimmeridgian suggest episodes of oxidized terrigenous matter. The same sequence of Callovian organic events is evident in much of the Early Cretaceous