Palaeogeography, Palaeoclimatology, Palaeoecology 302 (2011) 1–9

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Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r.com/ locate /pa laeo

Reconstructing mid- to high-latitude marine climate and ocean variability usingbivalves, coralline algae, and marine sediment cores from the Northern Hemisphere

Alan D. Wanamaker Jr.a,⁎, Steffen Hetzinger b,1, Jochen Halfar b

a Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011-3212, USAb CPS-Department, University of Toronto Mississauga, 3359 Mississauga Rd. N, Mississauga, ON, Canada L5L 1C6

⁎ Corresponding author. Tel.: +1 515 294 5142.E-mail address: adw@iastate.edu (A.D. Wanamaker)

1 Now at: Leibniz Institute of Marine Sciences (IFM24148 Kiel, Germany.

0031-0182/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.palaeo.2010.12.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 August 2010Received in revised form 23 December 2010Accepted 29 December 2010Available online 13 January 2011

Keywords:Marine climate changeBivalvesCoralline algaeSediment recordsHigh-resolution proxy records

Quantifying the role and contribution of the world's oceans in past, present, and future global change is anessential goal in climate, paleoclimate and environmental studies. Although the global oceans interact andinfluence climate greatly, the marine environment is substantially under-represented in key climateassessment reports, especially during the last millennium (IPCC, 2007; see Palaeoclimate chapter: 6.6—Thelast 2000 years). The under-representation of marine records in key climate documents likely results from theoften imprecise chronologies associated with many marine-based archives, which greatly hinders singularclimate comparisons (lag/lead phasing relationships) with well-dated, and/or annually-resolved archives.However, several marine archive records have excellent chronological constraint. In particular, many marinebivalve taxa and coralline algae have annual increments that formwithin their carbonate framework, that canbe used to establish an absolutely-dated chronology, via cross-dating techniques, from the marineenvironment. Additionally, in some cases, where sedimentation rates are high, and alternative chronologicaldating methods exist (e.g., tephrochronology) other than radiocarbon measurements (often greater than±40 years uncertainty), sediment archives can provide continuous, sub-decadal records of environmentalchange for centuries to millennia. This brief introductory article and accompanying special issue will focus onthe utilization of bivalves, coralline algae, and high-resolution marine sediment cores in paleoclimate andenvironmental studies within the most recent millennium with a focus on the Northern Hemisphere.

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1. Introduction and rationale

There is a critical need in climate studies to characterize the pastbehavior of the climate system on multiple time scales from a variety ofgeographic locations. Spatially and temporally diverse proxy-climaterecords can be used to elicit detailed information regarding the variousfactors that influence climate (Mann et al., 2009; Mayewski et al., 2004).Knowledge of those factors that may have been likely to initiate rapidclimate change events, especially within the last millennium, is criticallyneeded (IPCC, 2007; 4th Assessment Report, Intergovernmental Panel onClimate Change). The scarcity of such climate records, especially from theextratropical oceans, represents a serious knowledge gap in climatestudies, especiallywhen considering that rapid climate change transitionsare likely to have substantial impact on human populations and createsevere societal challenges in the future. Further, byproviding a frameworkof past marine climate variability, climate scientists and climatemodelerswill be better able to predict the impacts of anthropogenic activity(namely increased CO2 emissions) on the modern and future climate

system by utilizing high-quality proxy climate data in climate modelsimulations and comparisons (see McCarroll, 2010).

Given the spatial and temporal limitations of high-resolutionobservational records of past ocean conditions, reconstructions ofmarine environmental parameters have to rely on archival informa-tion, often derived from long-lived marine biota or sediment. Whilehigh-resolution proxy-based marine paleoclimate studies havetraditionally concentrated on the tropics, numerous proxy time serieshave been generated from mid- and high-latitude oceans during therecent decades. This is important because the extratropical oceans,particularly the North Atlantic, play an essential role in regulating theglobal climate system via deepwater formation and carbon storage.

In a recent review of paleoclimate archives, Jones et al. (2009a)discuss tree rings, corals, ice cores and historical information asmaking up the bulk of high-resolution climate proxy information.While speleothems, lacustrine and varved sediments are reviewed asadditional high-resolution archives, impressive recent advances inbivalve sclerochronology and to a lesser extent coralline algalsclerochronology are largely neglected. The latter archives, however,are able to fill the gap left by the spatially restricted occurrence ofreef-building coral proxy archives as they provide century-scale high-resolution climate information from extratropical regions. Formillennial-scale reconstructions extratropical marine sediments

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have yielded temporally extensive records; however, annual resolu-tion data can be extracted only in rare circumstances (e.g.,Baumgartner et al., 1992). While a variety of additional highlyresolved marine climate proxy archives exist from relatively shallowextratropical seas (e.g., otoliths, statoliths, and non-tropical shallowwater corals), this brief introduction and accompanying special issuewill focus on the utilization of bivalves, coralline algae, and high-resolution marine sediment cores in paleoclimate and environmentalstudies within the most recent millennium with a focus on theNorthern Hemisphere (see Figs. 1 and 2).

2. Ocean and climate records from bivalve mollusks

Shortly after the discovery of utilizing reef-building corals as seasurface temperature (SST) archives (Hudson et al., 1976) a number ofcoral-based temperature reconstructions were developed (e.g.,Dunbar and Wellington, 1981; Emiliani et al., 1978). At about thesame time, the potential of generating climate reconstructions frommid-latitude bivalve records was recognized by Jones (1981). Majoradvancements in bivalve sclerochronology and bivalve proxy devel-opment accelerated in the early 1980s (see Arthur et al., 1983; Jones,1980; Rhoads and Lutz, 1980) and steady progress has continued forthe last two decades resulting in several century-scale climatereconstructions from the North Pacific and the North Atlantic (e.g.,Black et al., 2009; Butler et al., 2009a; Schöne et al., 2003; Strom et al.,2004; Wanamaker et al., 2008a). At present, sclerochronologicalanalyses of bivalve mollusks supply the bulk of annual to sub-annualresolution extratropical marine climate data for near-surface watermasses.

The motivation to use bivalve records in climate and ocean studiesoriginates from their great utility as environmental recorders (seeRhoads and Lutz, 1980; Richardson, 2001). Bivalves are distributedglobally, inhabit a wide variety of environments and water depths,and their fossilized shells are abundant and widely available throughgeologic time (e.g., Krantz et al., 1987). Bivalves are often well-represented and often adequately preserved in archaeological sites,providing a powerful means for investigating past environments andcultures (e.g., Quitmyer and Jones, 2009; Quitmyer et al., 1997;Sandweiss et al., 2001; Walker and Surge, 2006). A number of bivalvespecies are extremely long-lived, with lifetimes of many decades oreven multiple centuries, some of which include freshwater pearlmussels (Schöne et al., 2004a), geoduck clams (Strom et al., 2004),ocean quahogs (Schöne et al., 2005a; Wanamaker et al., 2008b) anddeepwater oysters (Wisshak et al., 2009), making them idealcandidates for climate studies. Further, bivalves deposit internal growthincrements in their shells with tidal to annual periodicities (Clark, 1976;Jones, 1980; Richardson, 1989), thus paleo-environmental reconstruc-tions can be temporally constrained with sub-seasonal to annual

Fig. 1. Overviewmap of study sites, types of archives used, and respective studies included insites.

resolution (Jones and Quitmyer, 1996). In fact, with cross-datingtechniques developed in dendrochronology, master shell growthchronologies can be absolutely-dated and span several centuries ormore in length (Black et al., 2009; Butler et al., 2009a).

It has been shown that marine climate conditions such as seawatertemperature (e.g., Black, 2009; Black et al., 2009; Klein et al., 1996;Lazareth et al., 2003; Schöne et al., 2004b, 2005a; Strom et al., 2004;Wanamaker et al., 2008a; Weidman et al., 1994), seawater temper-ature seasonality/paleo-weather (e.g., Andreasson and Schmitz, 1996;Bojar et al., 2004; Goewert and Surge, 2008; Jones and Allmon, 1995;Patterson et al., 2010; Schöne and Fiebig, 2009; Schöne et al., 2005b),ocean upwelling (e.g., Andrus et al., 2005; Jones et al., 2009b, 2010),ocean 14C reservoir ages (e.g., Butler et al., 2009a; Jones et al., 2007;Wanamaker et al., 2008b; Weidman and Jones, 1993), atmosphere/ocean carbon dynamics (Butler et al., 2009a), productivity patterns(e.g., Wanamaker et al., 2009; Witbaard et al., 2003), and oceanic/atmospheric circulation patterns (e.g., Ambrose et al., 2006; Black,2009; Black et al., 2009; Lazareth et al., 2006; Müller-Lupp and Bauch,2005; Schöne et al., 2003; Wanamaker et al., 2008a; Weidman andJones, 1993) may be reconstructed from bivalve growth andgeochemical records. Recent advances in bivalve sclerochronology(and in the field of sclerochronology) will likely facilitate a morecomprehensive assessment of marine climate variability and globalchange issues, including anthropogenic impacts. For example,geochemical records (stable carbon isotope ratios) derived from theshells of marine bivalves (also sclerosponges and coralline algae)indicate that the increased burning of fossil fuels in the last 100 years,and the resultant CO2 emissions, have already noticeably changed thestable carbon isotopic composition (negative trend) of the surfacewaters in the oceans (see Böhm et al., 2002; Butler et al., 2009a; Swartet al., 2010; Williams et al., 2011). Additionally, a recent study using amarine sediment core collectedwithin Loch Sunart (Scotland) by Cageand Austin (2010) showed a pronounced negative shift in the stablecarbon isotopic composition of benthic foraminifera in the last100 years. An important note of the Cage and Austin (2010) study isthat they used other calcifying organisms (foraminifera), which arenot impacted by ontogenetic growth trends like bivalves. Althoughthe interpretation of stable carbon isotope ratios derived frombioarchives is often complicated by regional hydrographic variabilityand local productivity patterns, a global trend is emerging. Thisphenomenon is currently described as the δ13C Suess effect (afterSuess, 1953), and results from the admixture of isotopically negativecarbon derived from fossil fuels with Earth's modern atmosphericcarbon inventories. As expected, the changes in the stable carbonisotopic composition of oceanic dissolved inorganic carbon (δ13CDIC)pools, reflecting the atmospheric stable carbon isotopic trends(δ13CAtm), are slightly attenuated. Further, the magnitude and timingof the so-called δ13C Suess effect, as evidenced by marine proxy

this special issue. Shaded rectangles indicate study regions, black dots individual study

Fig. 2. Paleoclimate archives discussed in this issue. (A) Live- and dead-collected Arctica islandica shells from the Gulf of Maine. (B) High-resolution microscope photomosaic (froman acetate replica) of internal growth increments in A. islandica from the hinge region showing the juvenile phase (right), juvenile to adult phase (middle), and mature phase (left).Direction of shell growth is upward in each photo (C) Clathromorphum nereostratum growing attached to the seafloor at ~10 m water depth, Aleutian Islands, Alaska. (D) Digitized,high-resolution photomosaic of a coralline alga (C. nereostratum) from the Aleutian Islands, Alaska. Thin red lines represent digitally mapped annual growth lines. Uppermost layerrepresents year of collection. Black cavities are sporangial conceptacles (reproductive structures) forming annually. (E) Detail of sediment core section from a gravity core sampledon the north Iceland shelf. Laminations indicate deposition of tephra layers. Images from Wanamaker, Hetzinger, Halfar, and J. Eiríksson.

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records, is variable spatially, which is indicative of the spatiotemporalvariations in carbon fluxes and carbon sequestration in the globalocean (e.g., Grottoli and Eakin, 2007). The next logical step would beto use these data in carbon sequestration models (and data-modelcomparisons) to improve our collective understanding of how pastand present anthropogenic CO2 is being cycled and stored in theglobal ocean (see Quay et al., 1992; Swart et al., 2010).

2.1. Bivalve studies included in this issue

There are five papers in this issue using bivalve sclerochronologyto investigate ocean and climate dynamics. Four of these studiespresent information from the long-lived marine bivalve Arcticaislandica (ocean quahog) from the temperate North Atlantic, whileone study provides environmental data from the Arctic bivalveClinocardium ciliatum (hairy cockle) in the Barents Sea (see Fig. 1). Inthe study by Carroll et al. (2011-this issue), the authors use shellgrowth data from 22 live-caught C. ciliatum specimens from threestations in the Barents Sea to establish the impact of oceanograph-ically distinct water masses on shell growth, and to determine theinfluence of climatic forcing on ecological processes over decadalscales. The authors reported that bivalve growth was substantiallydifferent between populations largely living in Atlantic waters versusArctic waters (i.e., along the oceanic Polar Front), with bivalve growthrates highest in Atlantic water. In the study of Butler et al. (2011-thisissue), the authors investigated the stable carbon isotope (δ13Cshell)dynamics of A. islandica to determine if there were any consistentontogenetic effects on δ13Cshell values. The authors concluded that afterseveral decades of life, the δ13Cshell signature becomes stable and is notimpacted by age-related effects; hence the “relatively mature” δ13Cshellvalues are likely valid measures of ambient dissolved inorganic carbon(DIC) levels. On a similar note, Schöne et al. (2011-this issue;contribution 1) illustrated δ13Cshell values from A. islandica shellscollected from northern Iceland and the Gulf of Maine during theinterval of AD 1750 to AD 2003. The authors suggested that the long-term δ13Cshell values declined in correspondence with ambient

atmospheric δ13C trends (i.e., the oceanic δ13C Suess effect). In theGulf of Maine, Wanamaker et al. (2011-this issue) reconstructed pastseawater temperature seasonality estimates for intervals during the lastmillennium using A. islandica shells. The main finding reported by theauthors was a marked increase (~21%) in seawater temperatureseasonality during the early Little Ice Age as compared to Medievaltimes. Also utilizing the geochemical signature from A. islandica shells,Schöne et al. (2011-this issue; contribution 2) investigated the potentialof Mg/Ca and Sr/Ca ratios as seawater temperature proxies. The authorssuggested that after ontogenetic trends were removed from thegeochemical data, Mg/Ca and Sr/Ca ratios from shell material are viableseawater temperature indicators.

3. Ocean and climate records from coralline algae

Coralline algae are the most recently developed proxy archivediscussed in this issue. While coralline algae are distributed in marinehabitats from polar to tropical latitudes and from intertidal shores tothe deepest reaches of the euphotic zone (Nelson, 2009), they havereceived comparably limited attention, necessitating a brief overviewof their general occurrence and characteristics. Often they are adominant component of benthic communities and play a major role inthe ecology and development of most hard and soft substratesthroughout theworld (Adey andMacintyre, 1973; Kuffner et al., 2007;Steneck, 1986). Coralline algae can occur attached to a substrate or ina free-living mode of life, where they are called rhodoliths (Bosence,1983). In the tropics coralline algae form independent buildups, so-called algal ridges (Steneck et al., 2003), cement tropical coral reefs(Adey, 1998) and provide the sedimentary infill to reef frameworks(Dullo and Hecht, 1990). Outside the coral-reef belt coralline algae arethe most important framework builders (Freiwald and Henrich,1994). Similar to corals and bivalves, individual plants of corallinealgae can live for hundreds of years, while forming annual growthincrements in their calcified thallus (i.e., skeleton) (Frantz et al., 2005;Halfar et al., 2007). In contrast to bivalves, where sclerochronologicalinterpretations are complicated by a slowdown of growth with

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increasing shell age (Goodwin et al., 2003), corallines possess noontogenetic decrease of growth. A number of studies have nowdemonstrated that crustose coralline algae are well suited asrecorders of extratropical paleoenvironmental signals because they(1) are widely distributed in mid- and high-latitude oceans, (2) canlive for several centuries and (3) display well-developed growthincrements in a high-Mg calcite skeleton (Burdett et al., 2011-thisissue; Halfar et al., 2011-this issue; Hetzinger et al., 2011-this issue).

The feasibility of using a variety of species of both free-living,branching and encrusting, massive coralline algae as proxy archiveshas been confirmed during independent field calibration studies inthe temperate North Atlantic (Halfar et al., 2008; Kamenos et al.,2008). The massive growing coralline Clathromorphum sp. which iswidely distributed in both the subarctic North Pacific and NorthAtlantic can attain a thickness of more than 30 cm (Lebednik, 1976).As mean annual growth increment widths of Clathromorphum sp.range from 230–330 μm (Halfar et al., 2011-this issue; Hetzinger et al.,2011-this issue) multicentury-scale coralline algal derived climaterecords are possible.

3.1. Coralline algal temperature proxies

Early studies indicated that cyclic variations in the Mg-content ofcoralline algae are related to water temperature fluctuations (Chave andWheeler, 1965) and growth rates (Moberly, 1968). TheMg–temperaturerelationship was confirmed for temperate (Lithothamnion glaciale) andsubtropical (Lithothamnion crassiusculum) corallines by comparing Mg/Ca ratioswith oxygen isotopes and local SST (Halfar et al., 2000). UsingX-ray absorption near edge structure (XANES), Kamenos et al. (2008)demonstrated thatMg is bound to the calcite lattice of analyzed corallinealgae (L. glaciale and Phymatolithon calcareum) and therefore notassociated with organic components, further confirming the use ofcoralline algae as robust Mg-palaeotemperature proxies. This issupported by a calibration study using electron and ion microprobesthat found highly significant linear relationships between MgCO3 andSrCO3 (mol%) and SST (Kamenos et al., 2009). Similarly, significantcorrelations with late spring to late fall instrumental SSTwere found in a65-year long time series of electron microprobe-based Mg/Ca ratiosgenerated from Clathromorphum nereostratum collected in the BeringSea,Alaska (Hetzinger et al., 2009).Mg/Ca ratios in this sample relatewellto a 30-year δ18O time series measured on the same specimen. In theCanadian Atlantic, Mg/Ca ratios of Clathromorphum compactum wereshown to be positively related to both station-based and griddedinstrumental SST (Gamboa et al., 2010). The 116-year long Atlantic algalrecord suggests a variable negative relationship between Newfoundlandshelf SSTs and the North Atlantic Oscillation (NAO, see Jones et al., 1997)which is strongest after ~1960 and before the mid 1930s.

The oxygen isotopic composition of coralline algae has been thesubject of numerous studies since the 1960s when oxygen isotopeswere found to be significantly offset from so-called equilibrium values(Keith and Weber, 1965; Wefer and Berger, 1991). Rahimpour-Bonabet al. (1997) compared the geochemistry of modern cool-watercorallines with their tropical counterparts. They found that the δ18Ovalues of algae from temperate environments could be used as apaleotemperature proxy, because unlike their tropical counterpartsthey deposit carbonate in isotopic equilibrium with ambient seawa-ter. This, however, was not confirmed by Halfar et al. (2000, 2007,2008), who found a significant negative isotopic offset in corallinealgae from subarctic Newfoundland (L. glaciale), Gulf of Maine (C.compactum) and Alaska (C. nereostratum). However, Halfar et al.(2000, 2007) and Hetzinger et al. (2009) demonstrated a strongtemperature dependence of algal δ18O ratios by comparison withregional sea surface and air temperatures as well as Mg/Ca ratiosmeasured in the same specimens. A century-scale algal δ18O record (C.nereostratum) yielded the first annually-resolved shallow marineclimate reconstruction from the subarctic North Pacific/Bering Sea

region (Halfar et al., 2007). In this case, microscope-based growth-increment counting in combination with U/Th dating was successfullyused to confirm the age and the continuous growth of a 117-year oldspecimen of C. nereostratum (Halfar et al., 2007). The time seriesshows significant spectral power at frequencies typical for the ElNiño-Southern Oscillation bandwidth (4–5.5 years) and is signifi-cantly correlated to the Pacific Decadal Oscillation, the dominantclimate pattern in the northern Pacific (e.g., Mantua et al., 1997).

Coralline algal growth and cell calcification have the potential topreserve environmental signals as they are related to temperature andlight variability (Adey, 1970). Accordingly, Kamenos and Law (2010)analyzed cell-wall thickness in both, laboratory-grown specimens and50-year long time series of branched rhodoliths (L. glaciale). Resultsshowed that the thickness of cell-wall calcification in less extensivelycalcified cells within annual growth increments is negativelycorrelated to summer temperature. In addition, the time seriesindicates negative correlations between the Atlantic MultidecadalOscillation (AMO, see Delworth and Mann, 2000) and cell-wallthickness in both more- and less-extensively calcified cells for allbranches. However, Kamenos and Law (2010) observed no consistentrelationship between growth increment widths, temperature, cloudcover and the AMO.

The above summary shows that environmental information ispreserved in the abundant and ubiquitous coralline algae, yet thisarchive remains largely underutilized. Coralline algae have thepotential to yield numerous environmental records from regionswhere other long-lived biogenic proxy archives with annual resolu-tion are absent.

3.2. Coralline algal studies included in this issue

Specimens initially studied by Kamenos and Law (2010) werereinvestigated by Burdett et al. (2011-this issue). The authorsdemonstrate a weak statistically significant negative relationshipbetween less extensively calcified cells (summer growth) and wintercloud cover as well as annual and summer SST. From this, Burdett et al.(2011-this issue) present a cloud cover hindcast using summercalcification data and SST. The authors suggest a modest rise in cloudcover from 1910 to 2006. No consistent relationships between annualgrowth increment widths and temperature or cloud cover wereobserved in Kamenos and Law (2010) and Burdett et al. (2011-thisissue). In contrast, by combining growth increment width records ofmultiple modern and museum collected specimens of C. compactumfrom a broad geographic region in the subarctic northwest Atlantic,Halfar et al. (2011-this issue) produced a 115-year composite algalgrowth time series that is strongly related to regional SST patterns.Finally, a laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) study investigated the use of certain metal/Ca ratios (Mg/Ca, Sr/Ca, Ba/Ca, andU/Ca) aspaleotemperature proxies forNorthPacificand Atlantic corallines of the genus Clathromorphum (Hetzinger et al.,2011-this issue). While Mg/Ca ratios are strongly temperature con-trolled, correlations between Sr/Ca time series and SST data for bothstudy sites arepositivebutweak. Relationships betweenSST andU/Ca orBa/Ca are negative and largely statistically insignificant.

4. Ocean and climate records from high-resolution marinesediment archives

Marine sediments archive paleoenvironmental change in theoceans through time. During the past few decades paleoceanographicand climatic reconstructions have been conducted based on marinesedimentary records from all ocean basins. These efforts haveprovided unique insights into the changing states of the past oceansystem. Using sophisticated techniques, information on past SSTvariability can be obtained from the chemical composition orabundance of organisms contained in the sedimentary records, by

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analyzing the species abundance and faunal assemblage of planktonicforaminifera and diatoms (CLIMAP, 1976), oxygen isotopic composi-tions of planktonic and benthic foraminifera, or more recently, theratios of Mg/Ca and Sr/Ca in foraminiferal shells (Lea et al., 1999;Nürnberg et al., 1996; Rosenthal et al., 2004) or the ratio of certainorganic molecules (e.g., alkenones produced by coccolithophorids;Keigwin, 2002; Kennett et al., 2000; Müller et al., 1998;Weldeab et al.,2007).

By applying these paleoceanographic tools, SST records withtemporal resolutions ranging from decadal to centennial have beengenerated allowing an assessment of past ocean and climate dynamicsover the recent millennia, and changes in large-scale ocean circulation(e.g., Boyle and Keigwin, 1982; Bull et al., 2000; Keigwin, 2002). Suchreconstructions have provided a more detailed view on large-scalesurface ocean conditions during multi-century warm/cold episodes inthe Northern Hemisphere climate, which have been documented byother land-based climate archives. For example,major efforts have beenundertaken to better characterize distinct periods of change during thepast millennium, such as theMedieval Climate Anomaly (MCA) and theLittle Ice Age (LIA) (e.g., deMenocal et al., 2000; Eiríksson et al., 2006;Keigwin, 1996; Keigwin andPickart, 1999; Lundet al., 2006;Mann et al.,2009). However, many marine sedimentary proxy records often havebeen difficult to combine with annually-resolved reconstructions fromother high-resolution archives (e.g., bivalves, corals, tree-ring, ice core,and documentary sources) due to the commonly coarser temporalresolution (centennial- to millennial-scale), and larger uncertainties inunderlying age models/dating techniques.

Recently, an increasing number of marine sedimentary recordswith age control sufficient to reconstruct multidecadal-to-centuryscale variations have been produced, and collaborative efforts tofurther improve age models/dating techniques are underway (seeJones et al., 2009a). Annually laminated (“varved”) marine sedimentscan yield quantitative paleoclimatic reconstructions with annual andeven seasonal resolution (e.g., Romero et al., 2009; Sancetta andCalvert, 1988; Thunell et al., 1993). The preconditions for theaccumulation of these sediments are a seasonally heterogeneoussupply of sediments and a lack of physical or biological reworking(e.g., environments where bottom water oxygen content is persis-tently low enough to prevent burrowing organism from disturbingthe laminae) (Grimm et al., 1996). Such conditions are common insettings that are dominated by coastal upwelling or in sedimentarybasins with restricted circulation. Numerous studies have describedlaminated hemipelagic sediments from sites around the world incontinental margin settings including the basins off SouthernCalifornia (e.g., Santa Barbara Basin) (Behl and Kennett, 1996; Bullet al., 2000; Kennett and Ingram, 1995; Nicholson et al., 2006), theGulf of California (Barron et al., 2004; Calvert, 1966; Pike and Kemp,1997; Sancetta, 1995), the Saanich and Effingham Inlet (BritishColumbia) (Chang et al., 2003; Chang and Patterson, 2005; Dean andKemp, 2004; Sancetta and Calvert, 1988), and the Cariaco basin offVenezuela (Black et al., 2004, 2007; Haug et al., 2001; Hughen et al.,1996; Peterson et al., 1991).

At present only a small number of ultra-high resolution marinesedimentary archives have been retrieved that allow the reconstruc-tion of climate and ocean changes on seasonal to interannual scales(e.g., Baumgartner et al., 1992; Chang and Patterson, 2005; Dean andKemp, 2004). High-resolution sediment cores from the extratropicalNorth Atlantic have been used for reconstructions of marine surfaceconditions (Cage and Austin, 2010; Eiríksson et al., 2006; Jiang et al.,2005; Keigwin and Pickart, 1999; Keigwin et al., 2003; Knudsen andEiríksson, 2002; Kristensen et al., 2004; Richter et al., 2009). Becausethe north Icelandic shelf is situated near strong oceanographic andatmospheric climatic fronts, which separate temperate and Arcticconditions (Johannessen, 1986), it has been a focal point of marinesedimentary research providing exceptional archives capturingsurface ocean variability at interannual to decadal time scales (see

Sicre et al., 2008). Due to the distinct atmospheric and oceanicconditions along the oceanic Polar front, the region near Iceland ishighly sensitive to climatic changes, where even minor changes in thedistribution of water masses translate into major environmental/ecologic changes. These changes are then archived in high-resolutionmarine sediments. The north Icelandic shelf is characterized by aseries of sedimentary basins, some of which have extremely highsedimentation rates allowing sufficient temporal resolution forassessing past changes in oceanography and climate with highaccuracy (e.g., Knudsen et al., 2009). The environmental changes onthe Icelandic shelf have been intensively studied throughout the lastdecade, often with special emphasis on reconstructing the oceanicclimate of the last few millennia (e.g., Andrews and Giraudeau, 2003;Andrews et al., 2001; Eiríksson et al., 2000, 2006; Jiang et al., 2005,2007; Knudsen and Eiríksson, 2002; Knudsen et al., 2004, 2009; Ran etal., 2008; Sicre et al., 2008). The presence of well-known tephra layersfrom volcanic eruptions in Iceland has allowed the development ofmore precise tephro-chronological age models on marine cores (e.g.,Eiríksson et al., 2004, 2011-this issue; Larsen et al., 2002), thusreducing uncertainties associated with radiocarbon dating. Due tothese advantages, ocean sediment records from this region haveprovided a precise temporal constraint (within decadal accuracy) inthe northern North Atlantic with respect to the timing of the MCA/LIAtransition from the marine setting, which appears to have occurred at~AD 1300 (Sicre et al., 2008). Interestingly, these data from themarine environment and from compilations of glacial advances fromthe Swiss Alps (Holzhauser et al., 2005; Schaefer et al., 2009) suggestthat the onset of the LIA in the Northern Hemisphere occurred prior toa major reorganization in the North Atlantic Oscillation around AD1450 (e.g., Trouet et al., 2009). Hence marine records with highlyconstrained chronologies can offer important information regardingthe timing and phasing of climate change events between theterrestrial and marine environments.

4.1. Marine sediment studies included in this issue

Two studies in this issue use sediment records from the northIceland shelf to investigate paleoceanographic changes during the lastmillennium. In the study by Eiríksson et al. (2011-this issue), theauthors use two high-resolution sediment records to demonstrate acoupling of changes in marine reservoir ages and paleoceanographicshifts on the north Iceland shelf. To assess this relationship, deviationsbetween a tephro-chronological age-depth model and calibratedmollusk-based AMS radiocarbon age determinations are comparedto various high-resolution palaeoclimatic proxies (e.g., benthicforaminifera, ice-rafted debris). Eiríksson et al. (2011-this issue)report considerable changes in average reservoir ages through time,which the authors suggest is related to the changing influence of coldPolar and Arctic water masses onto the north Iceland shelf. Ran et al.(2011-this issue) reconstruct paleoceanographic changes on the northIceland shelf using precisely dated and high-resolution diatomrecords. The diatom-based record of reconstructed summer SST iscompared to instrumental and documentary data for the last 100 yearsto test its reliability. Temperature changes during the past millenniumare discussed with special focus on the MCA, the LIA, and the 20thcentury warming on the north Iceland shelf. In general, the diatom-based record from the north Iceland shelf shows high correspondenceto other proxy-based paleo-records in the North Atlantic region.

5. Conclusions and future outlook

A current challenge in climate and ocean studies is accessing andproviding annually-resolved and absolutely-dated records fromclimatically important regions in the oceans to characterize recentand past changes in ocean circulation, carbon cycle dynamics, andclimate change. Recent advancements in the field of sclerochronology

6 A.D. Wanamaker Jr. et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 302 (2011) 1–9

are beginning to contribute to this challenge. Using techniquesdeveloped in dendrochronology, sclerochronologists have nowdeveloped several cross-dated, annually-resolved, continuous mastershell chronologies of substantial length from the marine environment(see Black, 2009; Black et al., 2008, 2009; Butler et al., 2009a,b;Helama et al., 2006, 2007; Marchitto et al., 2000; Schöne et al., 2003;Scourse et al., 2006; Strom et al., 2004; Stott et al., 2010; Witbaardet al., 2003). Current and ongoing work utilizing coralline algae ashigh-resolution marine climate archives is promising. Coralline algaeare long-lived, globally distributed, and occupy a variety of marinehabitats and water depths; hence coralline algal records can be usedto reconstruct a wide-range of marine environments, includingregions were other bioarchives are absent. Additionally, high-resolution marine-based sedimentary archives from climaticallysensitive regions (e.g., near Iceland) that have excellent age/depthchronological constraints (e.g., tephro-chronological age models) areproviding key insights about the timing and magnitude of rapidclimate change events during the last millennium (see Eiríksson et al.,2011-this issue; Ran et al., 2011-this issue). These marine-basedrecords have the potential to substantially improve our understandingof past global climate change events, by more thoroughly represent-ing the ocean climate system.

Future research using bivalves, coralline algae, and high-resolutionmarine sediment proxy archives should not only include furtherproxy calibration with instrumental series, but should also involvemulti-proxy approaches using a combination of several archives.Helama et al. (2007) were one of the first to compare a master shellchronology (A. islandica) to a master tree-ring chronology (Scots pine;Pinus sylvestris) from northwest Norway. From this work, Helama etal. (2007) established that a modern, contemporaneous relationshipbetween shell and tree growth existed during strong NAO years.However, they concluded that the two proxies may have behaveddifferently in the past. Further, Black et al. (2009) combined geoduckclam growth data with tree-ring observations to create an improvedreconstruction of the Pacific SST. Additionally, Felis et al. (2010)combined geoduck clam growth data with coral-based geochemicaldata in the Pacific Northwest and reported that by combiningmultipleproxies from one region, the proxy-based compilation better reflectedthe major climate pattern (Pacific Decadal Oscillation) (also seeGedalof et al., 2002). In the Atlantic, marine proxy reconstructionsfrom the bivalve A. islandica and coralline algae were compared byHalfar et al. (2008). Such records are critical in assessing possibleanthropogenic-induced changes in oceanic, atmospheric, and terres-trial systems. At present, the majority of marine climate reconstruc-tions rely on time series that are based on individual records extractedfrom single specimens, which prevents the quantitative separation ofthe desired climate signals from noise at a given site (e.g., Jones et al.,2009b). Hence, similar to dendrochronology, future sclerochronologicstudies of mid- and high-latitude archives should focus on construct-ingmaster growth chronologies (with substantial replication) with anemphasis on usingmultispecimen approaches. Considering the recentadvancements in the construction of shell-based and coralline algalchronologies (see Butler et al., 2010; Halfar et al., 2011-this issue), itwill be possible to develop regional networks (e.g., North Atlanticnetwork, North Pacific network) similar to tree-ring networks, toevaluate regional responses to climate and ecosystem changes basedon master shell growth/coralline algae chronologies. Furthermore,such networks would allow a direct comparison between shell andtree-based records to determine temporal (lead/lag) relationshipsbetween the marine and terrestrial environments as a result ofclimate variability (see Black, 2009; Helama et al., 2007).

Acknowledgments

This paper and special issue resulted from a topical session at theEuropean Geosciences Union (EGU) General Assembly 2009 (Recon-

structing mid- to high-latitude climate and ocean variability fromhigh-resolution biogenic archives; CL19). We thank two anonymousreviewers for their thoughtful comments and suggestions.

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