An Overview of the Chemical Cycling of Pb (Lead) in the Global Ocean System
Daniel Hossfeld
The current understanding of lead cycling in the ocean includes human-dominated
sources and relatively short residence time due to the bonding capabilities of lead isotopes
(Noble, 2014 & Schaule, 1981). The study of lead in the oceans shows the impact that humans
can manage across just decades of changing inputs. Lead inputs into the atmosphere increased
dramatically in the industrial revolution, and peaked alongside the maximum leaded gasoline use
in the 1970’s (Noble, 2014). Sources of atmospheric lead includes gasoline, smelting, coal
burning, and incineration. Oceanographic surveys suggest that atmospheric deposition peaked
alongside leaded gas usage, and dipped sharply after a planned switch into unleaded fuels
(Nriagu, 1989). Concentrations range from 10​-12​ to 10​-4​ mol per kg of seawater (Bruland, 2003).
The undisputed major source of dissolved lead in the ocean is accumulated through
atmospheric dust deposition (Bruland, 2003). No other inputs are specified in publications.
According to Noble et al (2014), input rates decreased drastically from 1979 to 2011, although
input rates can only be estimated from measured dissolved lead in the water column. It is clear
that deposition has decreased across all lead isotopes since 1979, but location of deposition is
tied to high lead injection into the atmosphere. Many sites with higher lead deposition are
downwind of areas of high industrial activity. Input rates are highly variable, but Bruland et al
(2003) estimated current atmospheric input in the North Atlantic to be 820 picomoles per square
centimeter per year, compared with 14 pmol/cm​2​yr before industry. The Central North Pacific
and the Central South Pacific are estimated to have higher input rates compared with ancient
times as well, but are not as drastic (240:1.4 pmol/cm​2​yr & 14:1.4 pmol/cm​2​yr, respectively).
The major significant source of lead removal is the binding of lead isotopes to descending
particulates from the euphotic zone (Bruland, 2003). Detritus provides a removal source by
sinking through the water column, collecting lead, and landing in the sediment. Corals also
provide a pathway to removal by taking up small amounts of lead during growth (Noble, 2014).
Sedimentation provides a stable removal rate, and is a major influence on the decreasing levels
of lead since the reduction of leaded gasoline use. Removal rates are also only understood as
relative amounts through water column surveying. Passive scavenging distribution is the best fit
behavior for lead. Lead is not present in phytoplankton proteins- it is not required for the growth
of primary producers, it only binds opportunistically (Reuer, 2002).
An impressive result of the consistent surveying of the oceans is the visualization of
human-made lead pollution throughout the timescale of the industrial revolution. Fascinating ice
core studies in Greenland have found proof of Greek and Roman era increases in lead due to
small scale industrial increases (Hong, 1994). At certain stations, the water column shows high
concentrations of lead in the intermediate level relative to the surface layer and down to 1000m
(Noble, 2014). These high concentrations align with areas of high lead emissions during the
industrial revolution and subsequent reliance on leaded gasoline when modeled alongside wind
and atmospheric dust patterns. A point in accordance with this observation is the drastic decrease
in lead concentrations across ocean stations when moving geographically further away from
major emission zones (industrial cities) (Bruland, 2003). Surface lead concentrations change are
so dependent on geographic area that remote locations can be orders of magnitude lower than
regions nearby to high industrial lead output (Bruland, 2003).
Due to the varying amount of input across the survey period, residence times are not
certain and apparently depend on geographic location as well as available phytoplankton and
biological uptake. Oligotrophic surface waters are modeled to have a ~2 year lead residence time
due to active binding abilities with high phytoplankton concentrations, while deeper waters are
expected to be “decades to a century” (Noble, 2014 & Flegal, 1983). Scavenged-type dissolved
particles (lead included) are known to have short residence times (Bruland, 2003).
The two stations chosen for water column distribution observations are geographically
distant and provide an understanding of the different behaviors at distinct locations due to
variable atmospheric dust input. Both Figures 1 & 2 show scavenged-type distributions, but the
sites experience different atmospheric input and resultant divergent concentration levels
throughout the water column, although both show the characteristic decrease of dissolved lead
with depth. Differences in concentrations through the water column can be attributed to currents
bringing in lead from sites experiencing variable amounts of time without exposure to
atmospheric input.
Station ER-10 in the Indian Ocean shows high concentrations at the surface (~50
pmol/kg) with an exponential decrease throughout the water column down to stability at ~10
pmol/kg around 2000m (Figure 1). According to published sources, this distribution is
understood to reflect a site with relatively recent high lead inputs and low past inputs. The site
could be downwind of a high-output region. The sharp rate of decline is likely due to high
productivity, which is expected to allow lead to bind to sinking matter and exit the system
quickly.
Station 2 in the Atlantic Ocean shows intermediate concentration levels at the surface
(~30 pmol/kg) with a very slow rate of decline until 2000m where decline occurs rapidly (Figure
2). Concentrations are almost steady at 30 pmol/kg from the surface to 1000m depth, suggesting
a steady input of lead at this site for an extended period of time. This site could have experienced
less productivity than its Indian Ocean counterpart, which could explain a high level of
intermediate lead. An alternative reason could be very high input in the past, so much so that
lead was removed at a similar rate to the Indian Ocean station but after removal, concentration
remained high. The North Atlantic experienced high lead concentrations due to the downwind
location in relation to high leaded gasoline usage sites in the United States, so the second
suggestion is very possible. The sharp decline near 2000m could be a signal of a time before high
lead input, or a different current of water that had not seen surface deposition recently.
Lead is a known toxic substance in high concentrations and has documented effects on
humans, wildlife, plants and livestock (Demayo, 1982). The ocean is a perfect example of
humanity quickly passing natural process timescales and incurring environmental harm due to
industrial scale activities. While lead use has been curbed due to strict environmental regulations
in some locations, input still remains a problem and contamination remains a concern. Scientists
have heavily documented the contamination process, but further investigations into how to curb
lead emissions is a crucial part of remediation. While survey methods continue to improve, it is
possible to begin asking questions and predicting answers about future lead emissions. Modeling
can predict the effects of a revitalized coal economy in the United States, as well as other
industrial shifts expected from an anti-environment administration.
Figures
Figure 1. Indian Ocean lead distribution from Station ER-10, Surface to 5000m depth
Figure 2. Atlantic Ocean lead distribution from Station 2, surface to 2500m depth
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