Whales before whaling in the North Atlantic

It is well known that hunting dramatically reduced all baleen whale populations, yet reliable estimates of former whale abundances are elusive. Based on coalescent models for mitochondrial DNA sequence variation, the genetic diversity of North Atlantic whales suggests population sizes of approximately 240,000 humpback, 360,000 fin, and 265,000 minke whales. Estimates for fin and humpback whales are far greater than those previously calculated for prewhaling populations and 6 to 20 times higher than present-day population estimates. Such discrepancies suggest the need for a quantitative reevaluation of historical whale populations and a fundamental revision in our conception of the natural state of the oceans.     

Soluble and colloidal iron in the oligotrophic North Atlantic and North Pacific

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed "dissolved" concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that "dissolved" iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle.     

Phosphate depletion in the western North Atlantic Ocean

Surface waters of the subtropical Sargasso Sea contain dissolved inorganic phosphate (DIP) concentrations of 0.2 to 1.0 nanomolar, which are sufficiently low to result in phosphorus control of primary production. The DIP concentrations in this area (which receives high inputs of iron-rich dust from arid regions of North Africa) are one to two orders of magnitude lower than surface levels in the North Pacific (where eolian iron inputs are much lower and water column denitrification is much more substantial). These data indicate a severe relative phosphorus depletion in the Atlantic. We hypothesize that nitrogen versus phosphorus limitation of primary production in the present-day ocean may be closely linked to iron supply through control of dinitrogen (N2) fixation, an iron-intensive metabolic process. Although the oceanic phosphorus inventory may set the upper limit for the total amount of organic matter produced in the ocean over geological time scales, at any instant in geological time, oceanic primary production may fall below this limit because of a persistent insufficient iron supply. By controlling N2 fixation, iron may control not only nitrogen versus phosphorus limitation but also carbon fixation and export stoichiometry and hence biological sequestration of atmospheric carbon dioxide.     

Polychlorobiphenyls in North Atlantic ocean water

Concentrations of polychlorobiphenyls (PCB's) have been measured at the surface and at various depths in the water of the North Atlantic Ocean between 26 degrees N and 63 degrees N. The concentrations average about 20 parts per trillion and amount to an estimated 2 x 10(4) metric tons of PCB's in the upper 200 meters of water. The average concentrations of PCB's in the surface water of the Sargasso Sea are lower than those in the northern North Atlantic.     

Interannual variability in the North Atlantic Ocean carbon sink

The North Atlantic is believed to represent the largest ocean sink for atmospheric carbon dioxide in the Northern Hemisphere, yet little is known about its temporal variability. We report an 18-year time series of upper-ocean inorganic carbon observations from the northwestern subtropical North Atlantic near Bermuda that indicates substantial variability in this sink. We deduce that the carbon variability at this site is largely driven by variations in winter mixed-layer depths and by sea surface temperature anomalies. Because these variations tend to occur in a basinwide coordinated pattern associated with the North Atlantic Oscillation, it is plausible that the entire North Atlantic Ocean may vary in concert, resulting in a variability of the strength of the North Atlantic carbon sink of about +/-0.3 petagrams of carbon per year (1 petagram = 10(15) grams) or nearly +/-50%. This extrapolation is supported by basin-wide estimates from atmospheric carbon dioxide inversions.     

Dilution of the northern North Atlantic Ocean in recent decades

Declining salinities signify that large amounts of fresh water have been added to the northern North Atlantic Ocean since the mid-1960s. We estimate that the Nordic Seas and Subpolar Basins were diluted by an extra 19,000 +/- 5000 cubic kilometers of freshwater input between 1965 and 1995. Fully half of that additional fresh water-about 10,000 cubic kilometers-infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year. Patterns of freshwater accumulation observed in the Nordic Seas suggest a century time scale to reach freshening thresholds critical to that portion of the Atlantic meridional overturning circulation.     

Decline of subpolar North Atlantic circulation during the 1990s

Observations of sea surface height reveal that substantial changes have occurred over the past decade in the mid- to high-latitude North Atlantic Ocean. TOPEX/Poseidon altimeter data show that subpolar sea surface height increased during the 1990s, and the geostrophic velocity derived from altimeter data exhibits declining subpolar gyre circulation. Combining the data from earlier satellites, we find that subpolar circulation may have been weaker in the late 1990s than in the late 1970s and 1980s. Direct current-meter observations in the boundary current of the Labrador Sea support the weakening circulation trend of the 1990s and, together with hydrographic data, show that the mid- to late 1990s decline extends deep in the water column. Analysis of the local surface forcing suggests that the 1990s buoyancy forcing has a dynamic effect consistent with altimetric and hydrographic observations: A weak thermohaline forcing allows the decay of the domed structure of subpolar isopycnals and weakening of circulation.     

Radiocarbon variability in the western North Atlantic during the last deglaciation

We present a detailed history of glacial to Holocene radiocarbon in the deep western North Atlantic from deep-sea corals and paired benthic-planktonic foraminifera. The deglaciation is marked by switches between radiocarbon-enriched and -depleted waters, leading to large radiocarbon gradients in the water column. These changes played an important role in modulating atmospheric radiocarbon. The deep-ocean record supports the notion of a bipolar seesaw with increased Northern-source deep-water formation linked to Northern Hemisphere warming and the reverse. In contrast, the more frequent radiocarbon variations in the intermediate/deep ocean are associated with roughly synchronous changes at the poles.     

Eddy-driven stratification initiates North Atlantic spring phytoplankton blooms

Springtime phytoplankton blooms photosynthetically fix carbon and export it from the surface ocean at globally important rates. These blooms are triggered by increased light exposure of the phytoplankton due to both seasonal light increase and the development of a near-surface vertical density gradient (stratification) that inhibits vertical mixing of the phytoplankton. Classically and in current climate models, that stratification is ascribed to a springtime warming of the sea surface. Here, using observations from the subpolar North Atlantic and a three-dimensional biophysical model, we show that the initial stratification and resulting bloom are instead caused by eddy-driven slumping of the basin-scale north-south density gradient, resulting in a patchy bloom beginning 20 to 30 days earlier than would occur by warming.     

Tropical origins for recent North Atlantic climate change

Evidence is presented that North Atlantic climate change since 1950 is linked to a progressive warming of tropical sea surface temperatures, especially over the Indian and Pacific Oceans. The ocean changes alter the pattern and magnitude of tropical rainfall and atmospheric heating, the atmospheric response to which includes the spatial structure of the North Atlantic Oscillation (NAO). The slow, tropical ocean warming has thus forced a commensurate trend toward one extreme phase of the NAO during the past half-century.     

The deglacial evolution of North Atlantic deep convection

Deepwater formation in the North Atlantic by open-ocean convection is an essential component of the overturning circulation of the Atlantic Ocean, which helps regulate global climate. We use water-column radiocarbon reconstructions to examine changes in northeast Atlantic convection since the Last Glacial Maximum. During cold intervals, we infer a reduction in open-ocean convection and an associated incursion of an extremely radiocarbon ((14)C)-depleted water mass, interpreted to be Antarctic Intermediate Water. Comparing the timing of deep convection changes in the northeast and northwest Atlantic, we suggest that, despite a strong control on Greenland temperature by northeast Atlantic convection, reduced open-ocean convection in both the northwest and northeast Atlantic is necessary to account for contemporaneous perturbations in atmospheric circulation.     

Central North Atlantic Plate Motions over the Last 40 Million Years

The relative motion vector for the North American and African plates has been determined from detailed charting of the trend of the Atlantis fracture zone for over 1000 kilometers in the central North Atlantic near 30 degrees N and from identification of marine magnetic anomalies and deep-sea drilling results. The vector (pole) is located at 52.5 degrees N, 34 degrees W and has a magnitude (opening rate) of 5.7 x 10(-7) degree per year. Major changes in either the pole location or the opening rate are not evident for the last 40 million years.     

Corrected Late Triassic latitudes for continents adjacent to the North Atlantic

We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of approximately 0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today.     

On Pleistocene Surface Temperatures of the North Atlantic and Arctic Oceans

Two additional interpretations are given for the important data of D. B. Ericson on the correlation of coiling directions of Globigerina pachyderma in late Pleistocene North Atlantic sediments with ocean surface temperatures. One interpretation relates the distribution of this species to the distribution and circulation of ocean water masses. On the basis of our ice-age theory, our second interpretation uses the data and correlations of Ericson to establish temperature limits of a thermal node, a line on which glacial and interglacial temperatures were equal, for the North Atlantic Ocean. This line crosses the strait between Greenland and Scandinavia. Further, Ericson's interpretation of the 7.2 degrees C isotherm implies that the glacial-stage surface waters of the Arctic Ocean were between 0 degrees and 3.5 degrees C.