Particulate and dissolved ²¹⁰Pb activities in the shelf and slope regions of the Gulf of Mexico waters

Abstract

Concentration profiles of dissolved and particulate ²¹⁰Pb were measured on horizontal transects of the continental shelf and slope regions of the Gulf of Mexico. Vertical profiles in the slope station in the presence and absence of warm-core ring enable us to evaluate the differences in the scavenging intensity of ²¹⁰Pb. A comparison of the rate of production of ²¹⁰Pb in the overlying water column to that of its mean annual flux calculated from the sediment inventory allows us to get a better insight in to the lateral transport and deposition of ²¹⁰Pb to the slope sediments over a <100 year time scale.

Our results indicate that there is no general increase of dissolved or particulate ²¹⁰Pb concentration with depth. The scavenging residence time of ²¹⁰Pb for the entire 1500-m water column varied between 0.06 and 4.7 years. There was a strong correlation between the inventory of total (dissolved+particulate) ²¹⁰Pb and depth of the water column. The ratio of measured sediment inventory (F) of ²¹⁰Pb to its production (P) in the overlying water column varied between 0.69 and 1.63, suggesting that, at time, lateral inputs from boundary scavenging in a few sites are dominant, while in other places, vertical scavenging is the primary mechanism of transport for ²¹⁰Pb.

Introduction

One of the particle-reactive radionuclides in the ²³⁸U decay series is ²¹⁰Pb (half-life=22.3 yr), which has been successfully used to investigate oceanic processes related to scavenging and sedimentation. The primary sources of ²¹⁰Pb to the shelf and slope regions of the oceanic water column are the atmospheric fallout of ²¹⁰Pb to the surface water and production from the decay of its grandparent, ²²⁶Ra. In comparison, inputs of dissolved ²¹⁰Pb from rivers are small, mainly due to rapid scavenging of dissolved ²¹⁰Pb from the river waters and efficient trapping of river-borne sediments in the estuaries (Benninger, 1978). The residence time of ²¹⁰Pb in coastal and estuarine regions is <1 year, in contrast to the deep ocean where it is 30–100 years (Rama and Goldberg, 1961; Craig et al., 1973; Somayajulu and Craig, 1976; Baskaran and Santschi, 1993 and the references therein). This relatively longer residence time in the deep ocean can be utilized in the investigation of the differences in the scavenging intensity of Pb in different oceanic regimes. For example, ²¹⁰Pb has been utilized as a tracer for the investigation of transport processes on the continental shelf and slope regions (Bacon et al., 1988; Anderson et al., 1994; Biscaye and Anderson, 1994). Indeed, the first direct evidence of rapid scavenging of trace metals in the deep sea was inferred from the disequilibrium between ²¹⁰Pb and ²²⁶Ra (Craig et al., 1973). The deficiency of ²¹⁰Pb between its supply (in dpm m⁻²) and measured inventory (dpm m⁻²) in the water column is a measure of either the scavenging intensity or lateral transport in a given site. A comparison of the production rate (P) of ²¹⁰Pb to its flux (F), either derived from sediment inventory or sediment traps, will enable us to investigate sediment focusing and/or boundary scavenging of ²¹⁰Pb. The F/P ratio value of one is suggestive that the primary removal mechanism is by vertical scavenging while F/P>1 indicates additional sources of ²¹⁰Pb. Similarly, F/P ratios of <1 indicate lateral transport to other depocenters while values >1 will indicate lateral input from other areas. The combined effects of vertical scavenging (diapycnal) processes and lateral transport (isopycnal) can also be delineated from the measurements of ²¹⁰Pb in the water column, and the ²¹⁰Pb inventory in the underlying sediments.

There is a considerable body of literature describing water column profiles of ²¹⁰Pb from different ocean basins: Atlantic (North Equatorial Atlantic, Spencer et al. (1980); Northwest Atlantic, Cochran et al. (1987); Northeast Atlantic, Thomson et al. (1993); shelf and slope south of New England and in the Middle Atlantic Bight, Bacon et al (1988), Bacon et al (1994); Pacific, Nozaki et al. (1976); Chung and Craig (1983); North Pacific, Nozaki et al. (1980); Eastern South Pacific, Thomson and Turekian (1976); Indian Ocean (Central and Eastern Indian Ocean, Cochran et al. (1983); Western Indian Ocean, Chung (1987)); East China and Philippine Sea, Nozaki et al. (1990); Yellow Sea, Hong et al. (1999); and North Sea, Spencer et al. (1980). But data on ²¹⁰Pb where the water column as well as the sediments had been measured simultaneously are relatively few (e.g., Bacon et al., 1994). However, such combined measurements are needed to address the relative importance of vertical scavenging vs lateral transport.

We have measured the concentrations of ²¹⁰Pb in the sediments and water column in the Gulf of Mexico. The water sampling was carried out in those areas of the Gulf where either a warm-core ring was present or absent. These warm-core rings derived from the Gulf Stream have unique features such as low nutrients (Biggs, 1992; Biggs and Muller-Karger, 1994) and suspended particle concentrations (Baskaran et al., 1996), and relatively longer residence times of suspended particles and particle-reactive nuclides such as Th (Baskaran et al., 1996). Previous in situ and satellite observations have revealed that there are intense gradients in physical, chemical, and biological properties in the warm-core rings that are uniquely different from those outside. There are only few studies that had been carried out on vertical vs lateral removal of radionuclides within a warm-core ring (e.g., Baskaran et al., 1996).

In this paper, we report dissolved and particulate concentrations of ²¹⁰Pb in the water column, and sediment inventories of ²¹⁰Pb from selected sites in the shelf and slope regions of the Gulf of Mexico. The present study provides insight into the scavenging of Pb in a warm-core ring (more oligotrophic Caribbean water) and typical spring upwelling conditions. The specific questions that will be addressed include:

• How does the particulate ²¹⁰Pb concentration vary with depth? Is there any difference in the fraction of particulate ²¹⁰Pb in the presence and absence of a warm-core ring?

• How does the particle-water partition coefficient, K_d, vary with water depth in the presence and absence of a warm-core ring?

• How does the inventory of ²¹⁰Pb (dissolved and particulate) vary with depth?

• How does the residence time of Pb in the whole water column vary with total water depth?

• How does vertical and lateral transport of ²¹⁰Pb vary in the slope regions of the Gulf of Mexico waters?