Abstract
Ocean-floor sediment samples collected up to 150 years ago represent an important historical archive to benchmark global changes in the seafloor environment, such as species' range shifts and invasions and pollution trends. Such benchmarking requires that the historical sediment samples represent the state of the environment at - or shortly before the time of collection. However, early oceanographic expeditions sampled the ocean floor using devices like the sounding tube or a dredge, which potentially disturb the sediment surface and recover a mix of Holocene (surface) and deeper, Pleistocene sediments. Here we use climate-sensitive microfossils as a fast biometric method to assess if historical seafloor samples contain a mixture of modern and glacial sediments. Our assessment is based on comparing the composition of planktonic foraminifera (PF) assemblages in historical samples with Holocene and Last Glacial Maximum (LGM) global reference datasets. We show that eight out of the nine historical samples contain PF assemblages more similar to the Holocene than to the LGM PF assemblages, but the comparisons are only significant when there is a high local species' temporal turnover (from the LGM to the Holocene). When analysing temporal turnover globally, we show that upwelling and temperate regions had greatest species turnover, which are areas where our methodology would be most diagnostic. Our results suggest that sediment samples from historical collections can provide a baseline of the state of marine ecosystems in the late 19th century, and thus be used to assess ocean global change trends.
Highlights
Late nineteenth and early twentieth century oceanographic expeditions set out to explore the vast and widely unknown deep ocean
Historical samples were retrieved from the Ocean-Bottom Deposits (OBD) Collection held by The Natural History Museum in London
These two samples showed the lowest similarities in assemblage composition between neighboring Last Glacial Maximum (LGM) and Holocene assemblages (Figure 1A, gray dots), meaning that there was a greater change in planktonic foraminifera (PF) assemblage composition since the LGM in these two locations
Summary
Late nineteenth and early twentieth century oceanographic expeditions set out to explore the vast and widely unknown deep ocean. The voyage of HMS Challenger is a notable example As she sailed around the globe between 1872–76, researchers mapped for the first time the shape of the ocean basins and described over 4,500 new species of marine life (Manten, 1972). Coring techniques provide more accurate sediment chronology (e.g., Röhl et al, 2000); historical samples represent the seafloor environment as much as 50 years earlier than the earliest core samples collected (Wüst, 1964), and contain sediments without any objects deposited after 1900 These uncontaminated historical samples can be useful for chemical analyses of the seafloor (e.g., pollution trends; Dekov et al, 2010), single-specimen analysis (e.g., Reichart et al, 2003; Wit et al, 2010) and investigations of species range shifts and invasions in the past century (e.g., Hoeksema et al, 2011). It is important to assess the degree to which historical sediment samples represent Holocene or mixed-Pleistocene sediments
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