8 



SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES 



Likewise, a millennial understanding of past ocean climate 

 variability in the North Pacific-Bering Sea region is crucial to 

 the prediction of the impacts of anthropogenic climate change 

 on this rich ecosystem and on the role played by this region in 

 Northern Hemisphere climate at large. Links between the North 

 Pacific and North Atlantic climates clearly exist, but they are 

 poorly understood (Hetzinger et al., 2012). The Bering Sea, a 

 region transitional between Arctic, Subarctic, and Boreal regions 

 in the North Pacific (Adey and Hayek, 2011), is particularly sen- 

 sitive to warming because of its seasonal ice cover (Sigler, 2010); 

 continuation of the warming trend of the past decade (Overland 

 and Stabeno, 2004) will have a major impact on its fisheries 

 and seabird and marine mammal populations (Grebmeier et al., 

 2006). As in the Labrador Sea, limited instrumental climate data 

 for the North Pacific and Bering Sea are available, only from 

 1900; significant sea surface temperature (SST) information is 

 available only from the mid-1950s (Hetzinger et al., 2012). 



At present, schlerochronological analyses of the bivalve mol- 

 lusk Arctica islaudica provide the bulk of annual- to subannual- 

 resolution paleoclimate data for near-surface waters in the north- 

 western North Atlantic (Wanamaker et al., 2008, 2011). How- 

 ever, the northernmost confirmed occurrences of A. islaudica are 

 found along the southern shore of Newfoundland (Dahlgren et 

 al., 2000), which is the southern boundary of the core Subarctic. 

 Limited paleoproxy archives exist for the North Pacific, but most 

 high-resolution climate reconstructions are inferred from tree 

 ring chronologies and do not provide oceanic SST patterns (Hetz- 

 inger et al., 2011). Recent studies have shown that encrusting 

 coralline algae are high-quality recorders of extratropical paleo- 

 climatic signals because of their (1) abundance in mid- to high- 

 latitude oceans, (2) multicentury life span (Halfar et al., 2011a), 

 and (3) annual incremental growth patterns in a high-Mg calcite 

 framework that can be targeted for high-resolution geochemical 

 sampling (Halfar et al., 2007; Williams et al., 201 1). 



Clathromorphitm has an intricate anatomy and reproduc- 

 tion; the climate history of the late Holocene is written in its 

 yearly banding and calcite crystals. This study demonstrates 

 how an understanding of the biology, physiology, ecology, and 

 habitat geomorphology of two principal species of this genus is 

 essential to the further development of a high-resolution climate 

 archive for the Arctic and Subarctic. Reading this climate archive 

 requires not only an intimate knowledge of the interactions be- 

 tween climate and the complex linkages among the reproduc- 

 tion, growth, anatomy, and ecology of these two species but also 

 high-resolution analytical techniques that reveal the underlying 

 structural details that are the outcome of these interactions. 



Corallines as Climate Proxies 



Mg/Ca Ratios and Seawater Temperature 



The magnesium content of coralline algal skeletons re- 

 cords temperature changes (Figures 3, 4; Adey, 1965; Chave and 

 Wheeler, 1965). Such records have been greatly expanded (Hetz- 

 inger et al., 2009, 2011; Gamboa et al., 2010; Kamenos, 2011) 



using laser ablation-inductively coupled plasma-mass spectrom- 

 etry and electron microprobe analyses. Research currently un- 

 derway is producing data covering multicentury time scales. 



6a/Ca Ratios and Salinity 



The barium content of the calcium carbonate skeletons of 

 several marine calcifiers has served as a proxy for identifying 

 changes in the mixing of cold, nutrient-rich deep waters at the 

 surface (Lea et al., 1989; Fallon et al., 1999) or as a tracer for 

 riverine inputs. Chan et al. (2011) have shown that changes in 

 the Ba/Ca ratio in Clathromorphum indicate freshwater-induced 

 changes in ocean stratification. Analyses of this ratio provide a 

 proxy for assessment of salinity changes resulting from meltwa- 

 ter introduced into the Gulf of Alaska and the Alaskan Coastal 

 Current, as well as being an important source of information on 

 freshwater delivery into the Arctic Ocean. 



Growth as a Climate Archive 



A 115-year growth record from specimens of C. compactum 

 from Newfoundland and Quebec (Figure 5) produced a century- 

 long proxy archive for temperature in the northwest Atlantic 

 (Halfar et al., 201 lb). This chronology was successfully compared 

 to climate patterns of the Atlantic Multidecadal Oscillation. Halfar 

 et al. (201 la) linked information from yearly band widths with 

 cloud cover related to the Aleutian low-pressure system. Combined 

 with Mg as a proxy for temperature, a 640-year record of annual 

 growth for C. compactum in the Labrador Sea has been correlated 

 with sea ice cover (Halfar et al., unpublished manuscript). 



Isotopic Composition 



The oxygen isotope composition in the skeletons of Clath- 

 romorphum species records ambient seawater temperature. 

 Similarly, the high-magnesium carbonate content of these species 

 records 5"C values from the dissolved inorganic carbon in the 

 surrounding seawater. Williams et al. (2011) measured 5"C in 

 the coralline alga C. nereostratum to reconstruct the entry of 

 anthropogenic CO, into the northern North Pacific Ocean and 

 Bering Sea (Figure 6). From 1887 to 2003, the average decadal 

 rate of decline in 5"C values increased from 0.03% yr"' in the 

 1960s to 0.095% yr ' in the 1990s; this result is higher than ex- 

 pected from the 8'^C Suess effect. The authors concluded that an 

 increasing intensity of the Aleutian atmospheric low was respon- 

 sible for the upwelling of carbon-rich deep water. 



MATERIALS AND METHODS 



The collections utilized in this study were taken on scuba 

 dives based from small research vessels, which allowed bottom 

 surveys and assessments of the ecology and geomorphological 

 characteristics of cor-stromes. The research vessels were equipped 

 with laboratory facilities that allowed microscopic examination 



