Branwen Williams

Deep-water antipatharians: Proxies of environmental change

Deep-water (307-697 m) antipatharian (black coral) specimens were collected from the southeastern continental slope of the United States and the north-central Gulf of Mexico. The sclerochronology of the specimens indicates that skeletal growth takes place by formation of concentric coeval layers. We used 210 Pb to estimate radial growth rate of two specimens, and to establish that they were several centuries old. Bands were delaminated in KOH and analyzed for carbon and nitrogen stable isotopes. Carbon values ranged from -16.4‰ to -15.7‰; the oldest specimen displayed the largest range in values. Nitrogen values ranged from 7.7%c to 8.6‰. Two specimens from the same location and depth had similar 15 N signatures, indicating good reproducibility between specimens.

Freshening of the Alaska Coastal Current recorded by coralline algal Ba/Ca ratios

Arctic Ocean freshening can exert a controlling influence on global climate, triggering strong feedbacks on ocean‐atmospheric processes and affecting the global cycling of the world’s oceans. Glacier‐fed ocean currents such as the Alaska Coastal Current are important sources of freshwater for the Bering Sea shelf, and may also influence the Arctic Ocean freshwater budget. Instrumental data indicate a multiyear freshening episode of the Alaska Coastal Current in the early 21st century. It is uncertain whether this freshening is part of natural multidecadal climate variability or a unique feature of anthropogenically induced warming. In order to answer this, a better understanding of past variations in the Alaska Coastal Current is needed. However, continuous long‐term high‐resolution observations of the Alaska Coastal Current have only been available for the last 2 decades. In this study, specimens of the long‐lived crustose coralline alga Clathromorphum nereostratum were collected within the pathway of the Alaska Coastal Current and utilized as archives of past temperature and salinity. Results indicate that coralline algal Mg/Ca ratios provide a 60 year record of sea surface temperatures and track changes of the Pacific Decadal Oscillation, a pattern of decadal‐to‐multidecadal ocean‐atmosphere climate variability centered over the North Pacific. Algal Ba/Ca ratios (used as indicators of coastal freshwater runoff) are inversely correlated to instrumentally measured Alaska Coastal Current salinity and record the period of freshening from 2001 to 2006. Similar multiyear freshening events are not evident in the earlier portion of the 60 year Ba/Ca record. This suggests that the 21st century freshening of the Alaska Coastal Current is a unique feature related to increasing glacial melt and precipitation on mainland Alaska.

225 years of Bering Sea climate and ecosystem dynamics revealed by coralline algal growth-increment widths

Bering Sea climate and ecosystem dynamics have recently undergone major changes that have affected seasonal sea ice distribution and marine life, including commercially important salmon fisheries. Unfortunately, long-term Bering Sea dynamics are poorly understood, largely because of an absence of high-resolution marine proxy archives. Here we present the first record compiled from annual growth-increment widths of long-lived coralline algae collected in shallow-water habitats spanning the entire Aleutian Islands. While algal growth in the Aleutians exhibits a variable relationship with regional temperatures, it is strongly driven by changes in solar radiation reaching the seafloor. Therefore, it provides an exceptional archive of long-term light dynamics, which in the Bering Sea is attributed to changes in strength of the Aleutian Low (AL), the dominant climate pattern of the subarctic North Pacific. The AL is positively related to Bering Sea cloudiness and wind strength, which in turn fosters upper-ocean mixing. Mixing raises surface-water nutrient concentrations and stimulates plankton production, which is positively linked to Alaskan salmon abundance. Enhanced clouds and plankton production increase shading on the shallow seafloor and reduce algal growth. Light-driven algal growth rates track proxy-derived salmon abundance from 1782 onward, but are poorly related to temperature-dominated Pacific Decadal Oscillation (PDO) variability prior to the twentieth century. The algal record suggests that the present-day relationship of AL and PDO varied historically and that salmon stocks have been more closely related to AL strength via its effect on plankton abundance rather than PDO-related temperatures.

Multicentennial record of Labrador Sea primary productivity and sea-ice variability archived in coralline algal barium

Accelerated warming and melting of Arctic sea-ice has been associated with significant increases in phytoplankton productivity in recent years. Here, utilizing a multiproxy approach, we reconstruct an annually resolved record of Labrador Sea productivity related to sea-ice variability in Labrador, Canada that extends well into the Little Ice Age (LIA; 1646 AD). Barium-to-calcium ratios (Ba/Ca) and carbon isotopes (δ13C) measured in long-lived coralline algae demonstrate significant correlations to both observational and proxy records of sea-ice variability, and show persistent patterns of co-variability broadly consistent with the timing and phasing of the Atlantic Multidecadal Oscillation (AMO). Results indicate reduced productivity in the Subarctic Northwest Atlantic associated with AMO cool phases during the LIA, followed by a step-wise increase from 1910 to present levels—unprecedented in the last 363 years. Increasing phytoplankton productivity is expected to fundamentally alter marine ecosystems as warming and freshening is projected to intensify over the coming century.

North Pacific twentieth century decadal‐scale variability is unique for the past 342 years

Reconstructed sea surface temperatures (SSTs) derived from Mg/Ca measurements in nine encrusting coralline algal skeletons from the Aleutian archipelago in the northernmost Pacific Ocean reveal an overall increase in SST from 1665 to 2007. In the Aleutian SST reconstruction, decadal-scale variability is a transient feature present during the 1700s and early 1800s and then fully emerging post-1950. SSTs vary coherently with available instrument records of cyclone variance and vacillate in and out of coherence with multicentennial Pacific Northwest drought reconstructions as a response to SST-driven alterations of storm tracks reaching North America. These results indicate that an influence of decadal-scale variability on the North Pacific storm tracks only became apparent during the midtwentieth century. Furthermore, what has been assumed as natural variability in the North Pacific, based on twentieth century instrumental data, is not consistent with the long-term natural variability evident in reconstructed SSTs predating the anthropogenic influence.

Solution and laser ablation inductively coupled plasma–mass spectrometry measurements of Br, I, Pb, Mn, Cd, Zn, and B in the organic skeleton of soft corals and black corals

Proxy records can be derived from soft corals and black corals using minor and trace element measurements of the organic skeleton of these corals. Here, concentrations of Br, I, Pb, Mn, Cd, Zn, and B in the organic skeleton were determined using solution inductively coupled plasma–mass spectrometry (ICP-MS) in one black coral from 5 m depth and two soft corals from 85 and 105 m depth collected from a reef offshore of Palau in the western tropical Pacific. Solution ICP-MS results indicate that concentrations of some elements vary as expected with depth (Cd and Mn) while others are taxa specific (I) or colony specific (Br, Pb, Zn, and B). The intensities of the same elements normalized to 13C were also measured at high resolution using laser ablation (LA) ICP-MS along radial transects covering the lifespan of the colonies. The results here indicate that high-resolution LA ICP-MS elemental records in black corals could be more fully developed for paleoceanographic reconstructions. In contrast, results of the laser transects from the two soft corals were not reproducible for any of the elements, and no discernible patterns were detected that could be developed into reliable proxy records using the current LA ICP-MS method.

Historical Contaminant Records from Sclerochronological Archives

A number of marine and freshwater organisms, including scleractinian and proteinacious corals, coralline algae, sclerosponges, and bivalve mollusks, secrete skeletons that grow larger over time and may record environmental contaminants over the lifespan of an individual. Most of these organisms also form periodic growth patterns (growth increments, lines or bands) that can be used to accurately date contaminant archives produced from chemical or physical analysis of sequential skeletal samples (termed sclerochronology). The majority of records produced from these organisms thus far have focused on paleoclimate reconstructions, but there is a vast potential for information on changes in contaminant levels over time. Importantly, sclerochronological archives offer the potential for pre-anthropogenic baselines of naturally occurring substances to estimate the magnitude of anthropogenic pollution. This chapter presents an overview of existing contaminant records and discusses the limitations and potential for future work using archives derived from marine organism skeletons.

Does the Coralline Alga Leptophytum foecundum (Kjellman) Capture Paleoenvironmental Variability in the Arctic Ocean

Abstract Records of high resolution climate variability in the past are essential to understanding the climate change observed today. This is particularly true for Arctic regions, which are rapidly warming. Prior to instrumental data, proxy records can be extracted from high-latitude climate archives to provide critical records of past Arctic climate variability. Here, we investigate the feasibility of extracting records of climate and environmental variability from the skeleton of the crustose coralline alga Leptophytum foecundum from offshore the Sagavanirktok River in the Beaufort Sea. Although this alga forms an annually banded skeleton, age chronologies were established with difficulty due to the large uncalcified reproductive structures relative to low annual growth rates. Average measurements of skeletal Mg content, &dgr;18Oalga values, and &dgr;13Calga values were consistent among the analyzed specimens, but time series of these parameters only significantly correlated between two of the collected specimens for &dgr;18Oalga. No clear trends in environmental variability explained the patterns in the skeletal geochemistry over time. This suggests that ambient seawater combined with freshwater from the Sagavanirktok River drives the geochemistry of L. foecundum at this site. Thus, coralline algal specimens located near variable sources of low-salinity waters are not ideal organisms to use as proxy archives.