Biogeochemical analysis of ancient Pacific Cod bone suggests Hg bioaccumulation was linked to paleo sea level rise and climate change

Abstract

Deglaciation at the end of the Pleistocene initiated major changes in ocean circulation and distribution. Within a brief geological time, large areas of land were inundated by sea-level rise and today global sea level is 120 m above its minimum stand during the last glacial maximum. This was the era of modern sea shelf formation; climate change caused coastal plain flooding and created broad continental shelves with innumerable consequences to marine and terrestrial ecosystems and human populations. In Alaska, the Bering Sea nearly doubled in size and stretches of coastline to the south were flooded, with regional variability in the timing and extent of submergence. Here we suggest how past climate change and coastal flooding are linked to mercury bioaccumulation that could have had profound impacts on past human populations and that, under conditions of continued climate warming, may have future impacts. Biogeochemical analysis of total mercury (tHg) and δ13C/δ15N ratios in the bone collagen of archeologically recovered Pacific Cod (Gadus macrocephalus) bone shows high levels of tHg during early/mid-Holocene. This pattern cannot be linked to anthropogenic activity or to food web trophic changes, but may result from natural phenomena such as increases in productivity, carbon supply and coastal flooding driven by glacial melting and sea-level rise. The coastal flooding could have led to increased methylation of Hg in newly submerged terrestrial land and vegetation. Methylmercury is bioaccumulated through aquatic food webs with attendant consequences for the health of fish and their consumers, including people. This is the first study of tHg levels in a marine species from the Gulf of Alaska to provide a time series spanning nearly the entire Holocene and we propose that past coastal flooding resulting from climate change had the potential to input significant quantities of Hg into marine food webs and subsequently to human consumers.

Fish vertebrae as archeological biomarkers of past marine ecological conditions: Comparison of mercury levels in Chilean swordfish between the Middle Holocene and the modern period

“…Diagenesis refers to changes in the chemical constituent of tissue after deposition in sediments, which is always a concern in trace element and stable isotope analysis of archeological bone. However, previous studies have provided results on the level of mercury in bones or other biological remains (hair) of animal species (fish and mammals) from different archeological periods (Emslie et al, 2019; Gerlach et al, 2006; Halffman, 2009; Murray et al, 2015) and have demonstrated the reliability of mercury measurement and its biogenic origin. These authors mention that it is unlikely that the Hg levels in the remains would be affected by diagenesis, either by a decline or intrusion, especially because the concentration ranges are similar between ancient and modern samples.…”

Mercury in tunas and blue marlin in the North Pacific Ocean

“…In the North Atlantic, which has declining mercury concentrations in water in response to environmental regulations in North America and Europe, Cross et al and Lee et al documented declines in mercury concentrations of bluefish ( Pomatomus saltatrix ) and Atlantic bluefin tuna ( Thunnus thynnus ), respectively, captured from the Northwest Atlantic coast. Over the past 6000 yr, mercury concentrations in bones of Pacific cod ( Gadus macrocephalus ; recovered from a small island in the Gulf of Alaska) have fluctuated in sync with climate changes , with the highest concentrations occurring when glacial melting and sea‐level rise caused coastal flooding that is speculated to have increased the flux of mercury and methylmercury from land to water. In the modern record for the North Pacific, where mercury concentrations in water are increasing as a result of export of atmospheric mercury from Asia , increasing trends of mercury concentrations have also been observed in yellowfin tuna ( Thunnus albacares ) and in black‐footed albatross ( Phoebastria nigripes ) .…”

An awkward contaminant: are mercury concentrations in historically collected fossils a result of storage conditions?

“…Therefore, environmental pollution by means of mercury may result in human diseases (Harada 1995). An increased mercury content was described from historic human bone remains (Yamada et al 1995, Rasmussen et al 2008 and in a Pleistocene Pacific cod (Murray et al 2015), but not in older fossils. Therefore, the bioaccumulation and biomagnification processes may potentially be considered as a possible origin of such a significant amount of mercury in the examined fossil material.…”