David Alberto Salas-de-León

Role of Near-Bottom Currents in the Distribution of Sediments within the Southern Bay of Campeche, Gulf of Mexico

Abstract The sediment distribution in the southern Bay of Campeche is mainly governed by wind-induced currents and mesoscale cyclonic eddies. An increase in sediment grain size toward the sea is attributed to the presence of a convergence zone that sorts the sediments from heavier to lighter. A small anticyclonic eddy near the coast leads to a concentration of sediments westward of the Grijalva-Usumacinta River. We combined current profiles and charts of the distribution of magnetic susceptibility within sediments with a hydrodynamic numerical model to elucidate the distribution of the major sedimentary units in the Southern Bay of Campeche, Gulf of Mexico. Acoustic Doppler current profiles and geostrophic velocity data were used to set the water velocity at the open boundaries of the applied numerical model. The model was forced by the dominant winds. Sediment on the southeastern side of the bay is dominated by carbonates because of a lack of river discharge and other systems that contribute terrigenous sediments. In the southernmost side of the bay, where the major rivers are located, the bottom sediments are mainly of terrigenous origin.

Studies on picophytoplankton in the southern Gulf of Mexico: pigment analysis and potential importance of the picoeukaryote Prasinophyte Micromonas pusilla

Abstract Two coastal zones (influenced by the Coatzacoalcos and Grijalva-Usumacinta rivers) and an oceanic one (Campeche Canyon), from the southern Gulf of Mexico, were sampled during the ‘nortes’ or windy (February, 2004) season, to study the concentration and distribution of the phytoplanktonic pigments and the picoeukaryote fraction. Vertical profiles of accessory pigments indicated the presence and potential importance of autotrophic picoplankton in the waters of the Southern Gulf of Mexico. These pigments showed different distributions, associated with different hydrographic conditions between the oceanic zone and the coastal regions (e.g. presence and depth of a thermocline), indicating contrasting phytoplankton communities. The highest concentrations of accessory pigments (fucoxanthin, zeaxanthin, divinyl-chlorophyll a) were associated with shallow coastal zones (above 1% PARI), except for prasinoxanthin that also showed high concentrations in the Campeche Canyon. Pigments associated with prokaryotic picoplankton (zeaxanthin, divinyl-chlorophyll a) were more distinguishable in the Campeche Canyon than in the coastal zones. Chlorophyll a was not directly related to accessory pigments in most of the stations studied. Relatively high cell abundances of Prasinophyte Micromonas pusilla occurred at one coastal zone station, using molecular probes (FISH-TSA), and revealed the potential importance of this Prasinophyte in the picoeukaryote fraction from the study area, providing a new insight into their ecological role, relative to diatoms and coccolithophorids.

Differential distribution of diatoms and dinoflagellates in a cyclonic eddy confined in the Bay of La Paz, Gulf of California

The differential distribution of diatoms and dinoflagellates in the Bay of La Paz, Gulf of California, Mexico, was analyzed in summer of 2009, when a cyclonic eddy confined in the bay dominated the circulation. An uplift of the nutricline in the eddy drove high concentrations of nutrients to the euphotic layer. A differential phytoplankton distribution was observed to be associated with the eddy: there was an abundance of dinoflagellates close to the center of the cyclonic eddy, whereas diatoms were more abundant at the periphery. A significant inverse correlation (R = −0.62, p < 0.002) was found between the temperature at 25 m depth and the dinoflagellates abundance. Based on the temporal evolution of chlorophyll measured by MODIS satellite images, and a conceptual model proposed for the lifecycle of eddies, the cyclonic eddy may have been an old decaying structure. The effect of the cyclonic eddy on the phytoplankton distribution in this small semienclosed region was apparently similar to that found in larger eddies in the open ocean, but this is the first time such a differential distribution has been found associated to a confined eddy.

Effect of Environmental Factors on Zooplankton Abundance and Distribution in River Discharge Influence Areas in the Southern Gulf of Mexico

From April 8th to 19th 2010 we studied the hydrodynamic patterns and zooplankton biomass in the Coatzacoalcos River discharge influence area to determine the zooplankton biomass, abundance, and distribution, and its dependence on physical variables. The physical variables were temperature, salinity, relative density (σT) and the speed and direction of currents. Zooplankton biomass was obtained from organism samples taken using zooplankton close-open systems and absolute intensity acoustic data from an Acoustic Doppler Current Profiler (ADCP). The Coatzacoalcos River forms an anticyclonic eddy to the east of the plume and a smaller cyclonic eddy is formed to the west. Currently, velocities (103 mm s−1) measured using the ADCP indicate a highly dynamic area. The vertical positions of the thermocline, pycnocline and halocline indicate that they are modulated by a cold, wind-mixed layer. Zooplankton biomass, abundance and distribution were modulated by physical parameters including salinity, which affected the temporal and spatial distribution of zooplankton biomass. The largest zooplankton biomass concentrations were sampled near the coast and to the west of the Coatzacoalcos river plume, in the mixed base layer.

Persistent millennial-scale climate variability in the eastern tropical North Pacific over the last two glacial cycles

High-resolution sediment records from the eastern tropical North Pacific (ETNP) spanning the last ~240 ka B.P. were studied to document the nature of millennial-scale climatic events in the tropical Pacific and to investigate teleconnection mechanisms. We present organic carbon (%OC) and diffuse spectral reflectivity records as indicative of upwelling and productivity changes off NW Mexico over the middle to late Pleistocene. The new productivity records document the persistence of abrupt millennial-scale changes over the last two glacial cycles. Detailed spectral and wavelet time series analyses show the predominance of longer climatic cycles (2–6 ka) during the last and the penultimate glacial periods. The persistence of millennial variability during the penultimate glacial, in absence of large ice rafted debris events in the North Atlantic, suggests that freshwater input through ice sheet dynamics is not essential for millennial-scale climate variability. Given the worldwide emerging picture of remarkable similar millennial-scale records over long time periods, we suggest that the pacing of this climate variability may represent a natural resonance in the climate system, amplified by a tightly coupled oceanic and atmospheric teleconnection processes. We present a schematic scenario of millennial-scale climate change depicting the role of the tropical Pacific in this global teleconnection system by linking productivity and upwelling changes in the ETNP with shifts in the position of the Intertropical Convergence Zone and the strength of the subtropical North Pacific High.