Murray, M., McRoy, C., Duffy, L., Hirons, Am., Schaaf, J., Trocine, R., Trefry, J., (2015). Biogeochemical analysis of ancient Pacific Cod bone suggests Hg bioaccumulation was linked to paleo sea level rise and climate change. Fronteirs in Environmental Science. Vol3 doi: 10.3389/fenvs.2015.00008
Pacific cod (Gadus macrocephalus) plays a vital role in the Gulf of Alaska. Paific cod are widely distributed and hold ecological and commercial value. Cod’s importance stretches back over 7,500 years ago when initial human settlements relied heavily on marine resources. Murray et al., 2015 tested preserved, ancient Pacific cod bones at an archaeological site that dates back to the Holocene—an era of deglaciation (melting glaciers) and sea level rise in Alaska that was responsible for carving out present day continental shelves. Through biogeochemical analysis, the authors were able to study mercury (Hg) levels across almost the entire Holocene in one species—the first kind in Alaska. The results of the study offers a glimpse into how climate change and sea level rise affected human populations in the past, and what forewarning it may hold in light of recent climate change.
The archaeological site in this study is situated on a small island within the Katmai National Park and Preserve in Alaska, and is dubbed XMK-030 (see Image 2). The sediments on the site range from 7600-4100 before present (BP) and are associated with remnants of human occupation such as shell and bone middens (a fancier term for garbage pile). Jaw bones from twenty different ancient Pacific Cod were recovered and sampled for both isotopes (δ13C and δ15N) and Hg, while additional bones were analyzed solely for either isotope content or Hg. Testing for δ13C and δ15N ratios can reveal if there was any change in marine production or food webs. For example, δ13C can be used to trace food sources, so changes in this value may represent changes in oceanic production. δ15N on the other hand, tracks a species’ position in the foodweb, so changes in this value may indicate changes in food web structure (For more detail on nitrogen isotopes, see this past oceanbites post). Sixty-three samples of modern Pacific Cod tissue from the same area were also tested for Hg. In order to extract collagen from bones, small amounts of bone were exposed to sound waves, demineralized in acid, heated, precipitated and freeze-dried. While the original paper can provide the full rundown, some interesting steps in testing for mercury involved washing bones with detergent, rinsing them with reverse-osmosis water, drying them in an oven, then subjecting them to a suite of chemicals in order to digest, dilute and reduce. Testing modern day cod tissues involved drying, digesting them with acid, and heated until soft tissues were dissolved.
Findings & Significance
Over time, δ15N values were constant meaning that the trophic level, or the position in the food chain of ancient cod did not change. However, δ13C values increased due to either higher phytoplankton productivity or transport of sediments rich in plant matter to the bottom of the sea suggesting that coastal flooding did indeed occur (see Image 3). The results of mercury levels across time were interesting. Mercury concentration in bones peaked in concert with the melting of glaciers and sea level in the early to mid Holocene (52-4600 BP) then decreased when sea levels became stable. The authors suggest that as sea level increased, large expanses of terrestrial areas along the coast became inundated with water. Mercury that occurs naturally within Alaska mountains and soils, and bound within plant material became submerged and therefore, available to enter into the marine food web.
Mercury in seafood may be on some people’s radar, but it may be served up fried, steamed and baked on more plates in the near future. Mercury is a persistent and toxic compound that is easily taken up through respiratory and digestive systems. As a persistent compound, it can build up or ‘bioaccumulate’ in individual organisms, and up through the food chain. High accumulation of mercury in the body can affect central nervous system, and hence behaviour, development and growth. The findings from this study are significant, especially in light of current climate change and imminent sea level rise. They can help us predict and mitigate mercury levels in wildlife, thus safeguarding ecosystem and public health.