Pacific cod (Gadus macrocephalus) as a paleothermometer: otolith oxygen isotope reconstruction

https://doi.org/10.1016/j.jas.2012.05.009Get rights and content

Abstract

Stable isotope studies are increasingly important for understanding past environmental and cultural developments along the North Pacific Rim. In this paper, we present methods for using Pacific cod (Gadus macrocephalus) otoliths as a paleothermometer using a case study from Kodiak Island, Alaska. The results of this study indicate that Pacific cod otoliths record variable paleoenvironmental conditions during the Little Ice Age. The broad distribution of Pacific cod and success in using the otoliths as a paleothermometer makes this method widely applicable to researchers working throughout the northern Pacific Rim.

Highlights

► We present method for using archaeological Pacific cod otoliths as a paleothermometer. ► We argue these are useful in archaeological contexts around the North Pacific Ocean. ► We show Pacific cod otoliths record variable temperature histories.

Introduction

Archaeological fish otoliths, or “ear stones”, are increasingly used as paleothermometers because the ratio of stable oxygen isotopes deposited in these carbonate structures varies with water temperature (Devereux, 1967; Urey, 1947). This ratio can be used to estimate the environmental conditions where a fish was living. In an archaeological context, otolith stable isotope records have been produced for fish species in several regions (Andrus et al., 2002; Hufthammer et al., 2010; Rowell et al., 2010; Walker and Surge, 2006; West et al., 2011; Wurster and Patterson, 2001). These studies demonstrate that fish otoliths can be used to reconstruct ancient trophic positions, fresh and ocean water temperatures, and as indicators of climate change. Unlike other methods of paleoenvironmental reconstruction, like tree rings or glacial chronologies, archaeological otolith studies provide a direct link among prehistoric fish populations, the environment, and human behavior in the past.

Paleoenvironmental reconstruction based on archaeological material is integral to understanding the past along the Northern Pacific Rim. In particular, stable isotope studies have become important in dietary and climate reconstruction (Byers et al., 2011; Hirons et al., 2001; Misarti et al., 2009; Moss et al., 2006). Despite the recent interest in this methodology, as well as the central role that fish played in prehistoric economies in this region, studies of archaeological otoliths are limited. Previously, West (2009a) and West et al. (2011) used the stable oxygen isotopes in Pacific cod (Gadus macrocephalus) otoliths to reconstruct Little Ice Age (LIA) ocean conditions and to assess the relationship among climate, fish biogeography, and human foraging activity in the Gulf of Alaska. In this paper, we focus on the methods for using Pacific cod otoliths as a paleothermometer and suggest that the broad temporal and geographic distribution of this fish species makes their otoliths ideal for paleoenvironmental reconstruction in this region. Here, we outline this analytical method using a case study from Kodiak Island, Alaska.

Section snippets

Otoliths

Fish have three pair of otoliths (sagittae, asteriscae, and lapillae) that are used for acoustic perception and balance. Among these, the largest and most useful for oxygen isotope analysis in Pacific cod are the sagittae, which are well known to grow in daily, seasonal, and yearly bands (or annuli; Fig. 1). This growth can be seen in cross section as alternating translucent and dark bands: in Pacific cod, these represent periods of fast growth in the summer and slower growth in the winter.

Case study: the Karluk-1 site

The otoliths used in this study were sampled from the stratified and well preserved Karluk-1 site on Kodiak Island, Alaska, which has been described in detail elsewhere (Jordan and Knecht, 1988; Knecht, 1995; West, 2009b, 2011, Fig. 3). Kodiak Island is part of the Kodiak archipelago, which is located in the central Gulf of Alaska in the Northeastern Pacific Ocean. The Karluk-1 site is located at the mouth of the Karluk River on the southwest side of Kodiak Island and was excavated by Bryn Mawr

Identification and preparation

As described by West (2009a, 2009b) and West et al. (2011), Pacific cod otoliths were recovered from the bulk faunal samples excavated from each stratigraphic layer of the Karluk-1 site (Table 1). Each bulk sample was sifted through 1/8" screen and all otoliths were removed for analysis. The otoliths were graded based on preservation, and a total of 15 whole otoliths were selected from the deposits, representing all layers of the archaeological site over an approximately 500-year time period (

Results and discussion

Our analysis suggests that otoliths recorded variable mean environmental conditions over the last 500 years (West, 2009a; West et al., 2011, Fig. 4). Mean δ18O values range from 0.78‰ to 1.48‰, and the results have been plotted based on the median Accelerator Mass Spectrometry (AMS) date for each stratigraphic layer (Figure 4). There are two periods when δ18O values are relatively high, meaning ocean conditions were probably cooler: at the onset of occupation at 500 ± 30 BP or 530 cal BP and

Conclusions

The results presented above indicate that Pacific cod living around the Kodiak archipelago experienced changing environmental conditions during the last 500 years. In West (2009a) and West et al. (2011), we compared these data to other detailed paleoenvironmental records for the Gulf of Alaska, and these comparisons support the suggestion that cod tolerated some fluctuation in ocean temperature during the LIA.

Pacific cod have been recovered from archaeological sites throughout the region, so

Acknowledgments

This research is supported by the Prince William Sound Oil Spill Recovery Institute, the National Science Foundation, the Alaska Anthropological Association, and the University of Washington Quaternary Research Center. Thanks also to Koniag, Inc. and the Alutiiq Museum and Archaeological Repository for facilitating and supporting my use of the archaeological collections, and to Delsa Anderl and Dan Urban at the AFSC, the National Ocean Sciences AMS facility, and Dorinda Ostermann, Benjamin

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