Glacial-to-interglacial variations in the deep water at the Bermuda Rise inferred from a Nd isotope record covering the last million years

<p>The formation of North Atlantic Deep Water (NADW) in the North Atlantic is an important modulator of the climate system, as it drives the global termohaline circulation, responsible for the distribution of heat, salts and nutrients across the oceans. ODP Site 1063 (4584 m), on the deep Bermuda Rise, is located in the mixing zone between NADW and Antarctic Bottom Water (AABW) and appears to be a good location to study how ocean circulation and climate interconnect. Here we present a new record based on Nd isotope ratios that covers ~1 Ma at that Site. Our data shows Nd isotope ratios during parts of interglacials that are much lower than present day NADW. These results are coherent with recent published studies on the last interglacial–glacial cycle that show that the deep North Atlantic Nd isotope ratios are also lower than NADW during the early interglacial. However, Nd isotope values from the shallower DSDP Site 607 (3427 m), within the core of NADW, have remained similar to modern NADW during interglacials over the same time interval. Site 607 is thought to represent the deep North Atlantic, as shown by an Atlantic meriodional transect that displays Nd isotopes ratios for glacial and interglacial maxima over the last ~1 Ma. We suggest that Nd isotope ratios at Site 1063 do not fully represent the North Atlantic endmember of the AMOC during interglacials, but regional or local processes. However, glacial values at Site 1063 fitting those of Site 607 suggest that Nd isotope ratios represent, indeed, water mass mixing during glacial periods. The low Nd-isotope ratios in the deep Bermuda Rise during interglacials would be the result of particle-seawater exchange derived from the arrival of freshly ground, poorly weathered bedrock from the Canadian shield to the North Atlantic during major ice sheet retreats, such as deglaciations as well as stadial-to-interstadial transitions. Consequently, a deep, regionally constrained layer of seawater is tagged with this extreme Nd isotope signature that is not representative of the AMOC. We suggest that a benthic nepheloid layer, whose development is driven by a deep-recirculating gyre system regulated by the interaction between the northward flowing Gulf Stream and the southward flowing deep western boundary current, facilitates the periodical masking of the deep Atlantic Nd isotope signature at Site 1063. The intermittence of the masking allows for a speculation on how the deep-recirculating gyre system might have changed over the last ~1 Ma glacial-to-interglacial cycles.</p>

Quantifying the Role of Folding in Nonautonomous Flows: The Unsteady Double-Gyre

We analyze chaos in the well-known nonautonomous Double-Gyre system. A key focus is on folding, which is possibly the less-studied aspect of the “stretching+folding=chaos” mantra of chaotic dynamics. Despite the Double-Gyre not having the classical homoclinic structure for the usage of the Smale–Birkhoff theorem to establish chaos, we use the concept of folding to prove the existence of an embedded horseshoe map. We also show how curvature of manifolds can be used to identify fold points in the Double-Gyre. This method is applicable to general nonautonomous flows in two dimensions, defined for either finite or infinite times.

Low-Frequency Variability of Monsoon-Driven Circulation with Application to the South China Sea

The interannual variability of the upper-ocean circulation forced by seasonally varying monsoonal wind is investigated in a two-layer quasigeostrophic (QG) model, with the aim to understand the low-frequency variability of the South China Sea (SCS) circulation. It is demonstrated that the seasonally varying monsoonal wind can force the upper-ocean circulation with significant internal variability, which is mainly associated with the intrinsic nonlinear dynamics of the summer double-gyre system. This arises from the fact that the intrinsic variability, characterized by the Rossby wave adjustment in the winter single-gyre system, is much weaker than that in the summer double-gyre system driven by the intergyre eddy potential vorticity flux through barotropic instability.

Chaotic Behaviors in the Response of a Quasigeostrophic Oceanic Double Gyre to Seasonal External Forcing

In an oceanic double-gyre system, nonlinear oscillations of the ocean under seasonally changing external forcing are investigated using a 1.5-layer quasigeostrophic model and a simple model related to energy balance of the oceanic double gyre. In the experiments, the variable parameter is the amplitude of external seasonal forcing and the Reynolds number is fixed as 39, at which periodic shedding of inertial subgyres occurs. The authors found that entrainment (at 2 times the period of the forcing) and intermittency (on–off type), phenomena that are often seen in nonlinear systems, emerge with increasing amplitude of the forcing. They seem to be related to the generation mechanism and characteristics of long-term (from interannual to decadal) variations in the strong current region of subtropical gyres such as the Kuroshio and its extension region.

Transport in an Idealized Three-Gyre System with Application to the Adriatic Sea

Motivated by observations of surface drifters in the Adriatic Sea, transport in a three-gyre system is studied with the aid of dynamical systems techniques. Particular attention is paid to the issue of intergyre transport. The velocity field is assumed to be two-dimensional and incompressible and composed of a steady three-gyre background flow on which a time-dependent perturbation is superimposed. Two systems of this type are considered: 1) an observationally motivated, analytically prescribed model consisting of a steady background on which a multiperiodic time-dependent perturbation is superimposed, and 2) an observationally based model of the Adriatic Sea consisting of the mean surface circulation derived from surface drifter trajectories on which a time-dependent altimetry-based perturbation velocity field is superimposed. It is shown that for a small perturbation to the steady three-gyre background, two of the gyres exchange no fluid with the third gyre. When the perturbation strength exceeds a certain threshold, transport between all three gyres occurs. This behavior is described theoretically, illustrated using the analytic model and shown to be consistent with the observationally based model of the Adriatic. The relevance of the work presented to more complicated multiple-gyre problems is discussed.

Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: comparison between in situ and predicted data

In the frame of the BIOSOPE cruise in 2004, the spatial distribution and structure of phytoplankton pigments was investigated along a transect crossing the ultra-oligotrophic South Pacific Subtropical Gyre (SPSG) between the Marquesas Archipelago (141° W–8° S) and the Chilean upwelling (73° W–34° S). A High Performance Liquid Chromatography (HPLC) method was improved in order to be able to accurately quantify pigments over such a large range of trophic levels, and especially from strongly oligotrophic conditions. Seven diagnostic pigments were associated to three phytoplankton size classes (pico-, nano and microphytoplankton). The total chlorophyll-α concentrations [TChlα] in surface waters were the lowest measured in the centre of the gyre, reaching 0.017 mg m−3. Pigment concentrations at the Deep Chlorophyll Maximum (DCM) were generally 10 fold the surface values. Results were compared to predictions from a global parameterisation based on remotely sensed surface [TChlα]. The agreement between the in situ and predicted data for such contrasting phytoplankton assemblages was generally good: throughout the oligotrophic gyre system, picophytoplankton (prochlorophytes and cyanophytes) and nanophytoplankton were the dominant classes. Relative bacteriochlorophyll-α concentrations varied around 2%. The transition zone between the Marquesas and the SPSG was also well predicted by the model. However, some regional characteristics have been observed where measured and modelled data differ. Amongst these features is the extreme depth of the DCM (180 m) towards the centre of the gyre, the presence of a deep nanoflagellate population beneath the DCM or the presence of a prochlorophyte-enriched population in the formation area of the high salinity South Pacific Tropical Water. A coastal site sampled in the eutrophic upwelling zone, characterised by recently upwelled water, was significantly and unusually enriched in picoeucaryotes, in contrast with an offshore upwelling site where a more typical senescent diatom population prevailed.

Spatial variability of phytoplankton pigment distributions in the Subtropical South Pacific Ocean: comparison between in situ and predicted data

In the frame of the BIOSOPE cruise in 2004, the spatial distribution and structure of phytoplankton pigments was investigated along a transect crossing the ultra-oligotrophic South Pacific Subtropical Gyre (SPSG) between the Marquesas Archipelago (141° W–8° S) and the Chilean upwelling (73° W–34° S). A High Performance Liquid Chromatography (HPLC) method was improved in order to be able to accurately quantify pigments over such a large range of trophic levels, and especially the strongly oligotrophic conditions. Seven diagnostic pigments were associated to three phytoplankton size classes (pico-, nano and microphytoplankton). The total chlorophyll a (TChla) concentrations in surface waters were the lowest measured in the centre of the gyre, reaching 0.017 mg m−3. Pigment concentrations at the Deep Chlorophyll Maximum (DCM) were generally 10 fold the surface values. Results were compared to predictions from a global parameterisation based on remotely sensed surface TChla concentrations. The agreement between the in situ and predicted data for such contrasting phytoplankton assemblages was generally good: throughout the oligotrophic gyre system, picophytoplankton (prochlorophytes and cyanophytes) was a dominant class, the nanophytoplankton signature was also significant and relative bacteriochlorophyll a concentrations varied around 2%. The transition zone between the Marquesas and the SPSG was also well predicted by the model. However, some regional particularities have been observed where measured and modelled data differ. Amongst these features is the extreme depth of the DCM (180 m) towards the centre of the gyre, the presence of a deep nanoflagellate population beneath the DCM or the presence of a prochlorophyte-enriched population in the high salinity formation area of the South Pacific Tropical Water. A coastal site sampled in the eutrophic upwelling zone, characterised by recently upwelled water, was significantly and unusually enriched in picoeucaryotes, in contrast with the offshore upwelling site where a more typical senescent diatom population was dominant.