DIATOM RECORDS | Antarctic Waters

530 DIATOM RECORDS/Antarctic Waters

DIATOM RECORDS Contents Antarctic Waters Freshwater Laminated Sequences Large Lakes Marine Laminated Sequences North Atlantic and Arctic Pacific

Antarctic Waters C E Stickley, Norwegian Polar Institute, Tromsø, Norway J Pike, Cardiff University, Cardiff, UK V Jones, University College London, London, UK ª 2007 Elsevier B.V. All rights reserved.

Marine Antarctic Waters: The Southern Ocean Siliceous Ooze Belt The southern high latitudes play a critical role in the global climate system. Advances in understanding the paleoceanographic and cryospheric evolution of the Southern Ocean and Antarctica are helping us understand global climate change on a number of timescales. Diatoms are arguably the best fossil group for unraveling Antarctic climate change since they are prolific in the Southern Ocean (the area between the coast of Antarctica (,70 S) and the Subtropical Front (,40 S) (Fig. 1)) contributing up to 75% of Southern Ocean production, despite this being a high-nitrate, low-chlorophyll (HNLC) region. Such high diatom production in the surface waters combined with a high export flux to the seafloor makes the Southern Ocean the world’s main sink for biogenic opal (silica). In particular, the area beneath the Antarctic Circumpolar Current (ACC), a major link in the Thermohaline circulation, comprises a continuous belt of opal completely encircling Antarctica between 45 and 60 S (Fig. 2). Diatomaceous sedimentation and preservation has been occurring within this belt more or less continuously since at least Miocene times. Today the nutrient-rich Southern Ocean supports a unique flora comprising at least six planktonic endemic species according to Zielinski and Gersonde (1997): Eucampia antarctica, Fragilariopsis curta, F. kerguelensis, F. separanda, Thalassiosira gracilis and T. lentiginosa. Here, the Polar Frontal Zone (Fig. 1) acts as a major geographic barrier to the migration of planktonic diatoms separating cooler-water Antarctic species to

the south from warmer-water SubAntarctic species to the north. The fossil diatom record of the Quaternary obtained from recent sediments of the Southern Ocean is well-known, containing mostly, but not exclusively, extant species in high abundance. For example, Zielinski and Gersonde (1997) report diatom concentrations of 50–200  106 valves/g surface sediment in the Atlantic sector beneath the ACC. The so-called ‘siliceous ooze belt of well-preserved diatoms’, is bound north and south by sediments containing less well-preserved diatoms (Fig. 2); carbonate-rich sediments to the north, and silty diatomaceous clay to the south including the shelf regions. These boundaries, approximating to the modern Polar Front (north) and the mean position of the winter sea-ice edge (south), may have shifted north and south in the past during periods of climate change. Over the Antarctic continental shelf high-biogenic silica deposition also occurred in the Quaternary but records are more patchy, although there are regions where diatom productivity and flux were exceptionally high during the last deglaciation resulting in distinct seasonal layering (see Marine Laminated Sequences).

Applications of Quaternary Antarctic Diatoms Despite preservation issues (see Diatom Introduction and Marine Diatoms), marine diatoms are excellent tools for climate reconstruction because they accurately reflect the environment in which they live and are normally the first microplankton group to respond to environmental change. Their use as a paleoenvironmental tool is perhaps of most value for marine sediments recovered from below the calcium carbonate compensation depth (CCD) and in the high-latitude regions such as the Southern Ocean. In both of these environments, siliceous microfossils (diatoms, silicoflagellates, and radiolarians) prevail over calcareous microfossils (nannofossils and

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Figure 1 Oceanographic fronts and zones in the modern Southern Ocean. For references to the position of the fronts and zones and of the summer and winter sea-ice edges see abbreviations list. Black dots ¼ the location of the 228 surface samples represented in DD228 of Armand et al. (2005), Crosta et al. (2005), and Romero et al. (2005). Boxed regions represent regions of denser surface sample coverage, which are detailed in Armand et al. (2005). Reproduced from Armand et al. (2005) Palaeogeography Palaeoclimatology Palaeoecology 223: 93–126, with permission from Elsevier BV.

planktonic foraminifera) because carbonate is not preserved below the CCD (preservational advantage), and reproduction of calcareous-walled microplankton is severely inhibited in polar waters (environmental advantage). This means, for example, that Antarctic marine diatoms are arguably the most reliable microfossil group for reconstructing past Southern Ocean environments. Of critical interest are past fluctuations in sea-surface temperature (SST), circum-Antarctic sea-ice patterns, latitudinal shifts in oceanic fronts and their role in carbon dioxide draw-down. To this end, there has been significant recent progress in assessing modern species distribution patterns (biogeography) and in the refinement of Antarctic diatom-based statistical techniques, which has advanced our understanding of the main environmental drivers controlling species abundance. In this way, Antarctic diatoms are the key to elucidating the climatic and oceanographic history of the Southern Ocean. Furthermore, since most Quaternary Antarctic species are extant, ecologicalbased applications can be applied to the recent past

with confidence. Quaternary marine Antarctic diatoms are also widely used in biostratigraphy for both offshore regions and on the Antarctic shelf. Here, they can also help elucidate sea-level changes, past glacial processes, and ice sheet history. North of Antarctica, an interesting but somewhat underdeveloped application, for which they are unequivocal, is in their use for revealing Antarctic-source bottom water in the world’s oceans. The world’s richest source of biogenic opal has resulted in a large number of studies involving Quaternary Antarctic diatoms in the Southern Ocean. It is impossible to discuss all of them here or even to acknowledge only the pioneering and best-known works. Instead we summarize those reports which have widespread importance for Antarctic studies and suggest further reading, on these and other topics. From the Southern Ocean we focus on biogeography, SST, and sea-ice reconstructions (including frontal movements), bottom water tracers, and biostratigraphy. Diatoms from lake sediments and ice cores from the Antarctic continent are also briefly discussed.

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Figure 2 Siliceous ooze belt of well-preserved diatoms in surface sediments around Antarctica (dark blue within the shaded area). Poorly preserved diatoms are indicated by light blue dots. Reproduced from Burckle (1984) Marine Micropaleontology 9: 241–261, with permission from Elsevier BV.

Biogeography, Past Sea-Surface Temperatures and Sea-Ice Patterns Recent advances have been made in mapping the biogeography of modern marine diatoms in Antarctic seafloor sediments (i.e., core-top sediments representing modern or submodern conditions) with respect to environmental controls. This is perhaps one of the most valuable procedures for assessing which hydrographic and cryospheric parameters control the occurrence and abundance of species living in the overlying surface waters. These modern diatom datasets, or training sets, have been analyzed by a variety of statistical techniques and transfer functions have been created. Techniques such as the modern analog technique and the generalized additive model have enabled quantitative reconstructions of SSTs and seasonal sea-ice patterns for the Quaternary. These statistical methods are reproducible and follow

the assumption that the biogeographic variability of species preserved in the surface sediments reflect the surface water hydrology and, therefore, may be used as a proxy for paleoenvironmental variability. The merits of assessing past SSTs and sea-ice patterns are clear; the warming and cooling of the ocean surface is a primary gauge for climate change, while sea-ice (coverage, duration, and concentration) is especially important for climate models because of the enormous feedback effects and energy exchanges associated with its growth and decay, as well as its impact on the thermohaline circulation. As most diatom flux to the Southern Ocean sediments occurs during the austral summer, relationships between diatom distributions and environmental parameters are best derived with reference to summer conditions. However, reference to winter conditions is equally valid as it provides a means to contrast seasonal

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extremes as well as factoring in the controllers for particular species distributions. Biogeography Until the last decade or so, understanding of the relationship between the distribution of Antarctic diatoms in core-top sediments to surface water hydrology was limited to a few early reports from the 1960s and 1970s. Although pioneering, these reports accounted for either only a limited area of the Southern Ocean or covered a wider area in less detail. One of the more useful later reports is that by Burckle (1984). He mapped species distributions and preservation from core-tops in all sectors of the Southern Ocean, covering its entire latitudinal range from nearshore to just north of the present day position of the Subtropical Front (Figs. 1 and 2). Although the dataset was relatively small (55 core-tops; nine significant species) and the data points relatively scattered (Fig. 2), Burckle produced one of the first distributional studies to cover the entire circum-Antarctic and he attempted to statistically quantify the relationship between species distribution and environmental parameters. Later, Zielinski and Gersonde (1997) provided the first truly comprehensive modern species-specific distributional dataset for the Atlantic Sector of the Southern Ocean. They clearly showed that the biogeography and abundance of 35 significant diatom species in this region are closely related to summer SSTs and the presence or absence of sea-ice and, therefore concluded that distributional patterns clearly do have paleoenvironmental significance. A more extensive biogeographical dataset in terms of both coverage and detail is provided in a set of linked research papers by Armand et al. (2005), Crosta et al. (2005), and Romero et al. (2005). Their comprehensive modern database (termed Diatom Database 228 or DD228) incorporates the latest taxonomic concepts and wholly supports the use of fossil diatoms to reconstruct past environments. The application of this dataset will allow a more reliable interpretation of the downcore diatom signal in future Southern Ocean paleoceanographic studies. Appropriately, they broaden the use of the sea-ice control by taking into account its concentration (% water covered) and duration, not merely its presence or absence. This gives a much more complete view of the relationships between diatom distribution and environmental parameters. They covered a much wider area than previous studies, and the biogeography of 32 major diatom species/ taxa in 228 core-top sediment samples from all sectors of the Southern Ocean from the coast to just north of the Subtropical Front was mapped (Fig. 1). For example, the distribution pattern of the

Figure 3 Light microscope image (plane-polarized light) of the endemic diatom Fragilariopsis kerguelensis, one of the main contributors to biogenic opal in Southern Ocean sediments. Image taken from late Quaternary laminated sediments of the East Antarctic Margin. The lower specimen is 50 mm long. (Image by EJ Maddison, Cardiff University, UK.)

abundance of the open-ocean endemic diatom Fragilariopsis kerguelensis (Fig. 3), one of the main contributors to biogenic opal in the Southern Ocean, reveals that it occurs in highest abundance when SSTs are between 1 C and 8 C (Fig. 4). In DD228, each diatom species is defined as having most affinity with one of three zonally distinct regions: (1) the sea-ice zone between the maximum average winter and summer sea-ice edges, where sea-ice occurs annually, (2) the open-ocean zone between the maximum average winter sea-ice edge and the SubAntarctic Front, and (3) the tropical/subtropical zone north of the SubAntarctic Front. Although all 32 diatom taxa occur throughout the study region, each to a greater or lesser degree, the correlation of their geographic patterns with sea-surface parameters illustrates the principal environmental control on the occurrence and abundance of each species (see Table 1, sea-ice diatoms). For the sea-ice and open-ocean zones, these controls are summer (February) SSTs and sea-ice coverage in terms of (i) its concentration during seasonal extremes (summer and winter) and (ii) its annual duration (months/year). In the tropical/subtropical zone the main controllers are warmer SSTs and minimal influence by sea-ice. Past Sea-Surface Temperatures and Sea-Ice Patterns Past SSTs and sea-ice patterns should be considered together since they are interrelated, albeit not necessarily linearly, over long timescales. For example, Crosta et al. (2004) showed in a quantitative diatom study of the Indian sector of the Southern Ocean that during the transition from interglacials to

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Figure 4 Surface sediment distribution of Fragilariopsis kerguelensis relative abundances in DD228. See Figure 1 for oceanographic fronts and zones. Reproduced from Crosta et al. (2005) Palaeogeography Palaeoclimatology Palaeoecology 223: 66–92, with permission from Elsevier BV.

glacials, sea-ice expansion lagged the SST decrease by approximately 1 thousand years (ky), while sea-ice retreat was almost synchronous with the SST increase at glacial–interglacial terminations. Sea-ice (Fig. 5) provides an important and vast habitat for Antarctic diatoms, supporting rich assemblages throughout the growing season. At the height of their growth, populations of sea-ice diatoms can become so dense that they color the underside of the sea-ice a yellowy-brown color. In the austral winter sea-ice expands to an area of up to approximately 19  106 km2 (Fig. 1) on average, and although inhibiting surface water production and sedimentation, it can stimulate huge blooms of diatoms in the surrounding waters during the spring melt. So-called sea-ice-related taxa, such as the well-known bipolar diatom Fragilariopsis cylindrus (Fig. 6), have been used to trace winter (August) sea-ice patterns for the Quaternary. Crosta et al. (2004), for example, use F. cylindrus in their Sea Ice Group to quantitatively reconstruct the sea-ice duration of the Indian sector of the Southern Ocean for the last 220 ky using the modern analog technique. Estimations of past summer (February) sea-ice patterns are more problematic since no microfossils are preserved which allow

paleoenvironmental reconstruction. However, occurrences of the cold-water (year-round sea-ice) indicator F. obliquecostata, along with a fall in biogenic sedimentation, may indicate the sporadic presence of summer sea-ice. Although their dataset was never published in detail, Pichon et al. (1992) was one of the first studies to develop a transfer function for reconstructing past SSTs from diatoms in core-top material from the Atlantic and Indian sectors of the Southern Ocean and which factored in problems caused by dissolution. They applied their transfer function to two cores in the Indian sector of the Southern Ocean to produce SST estimates for the last ,140 ky. Later, Zielinski et al. (1998) developed a more rigorously tested SST transfer function for the same region providing summer SST estimates for a core located in the Atlantic sector over the same time interval. Other studies have concentrated on reconstructing conditions at particular time-slices such as the Last Glacial Maximum (LGM). For example, Crosta et al. (1998), revising the Pichon et al. (1992) dataset, were the first to use the modern analog technique to quantitatively reconstruct circum-Antarctic sea-ice patterns from marine diatoms in terms of months

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Table 1 Temperature and sea-ice parameter ranges and maximum relative abundances of the 14 sea-ice species and groups in DD228 (adapted from Armand et al. (2005) Palaeogeography Palaeoclimatology Palaeoecology 223: 93-126, with permission from Elsevier BV) Sea-ice diatoms (data from Table 1, Armand et al., 2005)

Actinocyclus actinochilus Chaetoceros resting spores Fragilariopsis curta Fragilariopsis cylindrus Fragilariopsis obliquecostata Fragilariopsis rhombica Fragilariopsis ritscheri Fragilariopsis separanda Fragilariopsis sublinearis Porosira glacialis Porosira pseudodenticulata Stellarima microtrias Thalassiosira antarctica group Thalassiosira tumida

Max. relative abundance (MRA) (%)

2.9

Feb. SST at MRA ( C)

0 to 1

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9

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82

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Figure 5 Multiyear sea-ice off the western Antarctic Peninsula in austral summer (December) 2003. (Photograph by CE Stickley taken aboard the RSS James Clark Ross of the British Antarctic Survey.)

per year (duration) at the LGM. In their comprehensive ‘state-of-the-art’ compilation of published and new datasets, Gersonde et al. (2005) provide the most up to date estimation of summer SSTs and sea-ice distribution patterns in the Southern Ocean at the LGM based on quantitative analysis of diatoms and radiolarians. Based on new data and reference

Figure 6 Secondary electron-scanning microscope image of a colony (eight frustules) of the bipolar diatom Fragilariopsis cylindrus, used to trace past winter sea-ice patterns in the circumAntarctic. Taken from late Quaternary laminated sediments of the East Antarctic Margin. Scale bar ¼ 20 mm. (Figure 4C of Stickley et al. (2005) for reference see Marine Laminated Sequences.)

datasets of several previously published works Gersonde et al. applied three statistical methods to 122 marine cores located in all sectors of the Southern Ocean to quantitatively reconstruct the glacial Southern Ocean at the EPILOG (Environment

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of approximately 39  106 km2, a northward expansion of 100% compared to the present, which agrees with earlier estimations (e.g., CLIMAP (Climate and Environment Monitoring with GPS Atmospheric Profiling)). However, unlike previous studies, they suggest an increased seasonality at the LGM based on the large differences between the extent of the winter and postulated summer sea-ice fields compared with those of today (Fig. 7). In addition, their

Processes of the Ice Age: Land, Ocean, Glaciers) time-slice (19.5–16.0 thousand years ago (ka) or 23–19 calendar years before present (cal ky BP). Their results indicate that at the EPILOG LGM the maximum winter sea-ice extent (at a concentration of >15%) reached as far north as nearly 47 S in the Atlantic and Indian sectors, but only to 57 S in the Pacific (Fig. 7). This equates to a total, asymmetric, EPILOG LGM circum-Antarctic winter sea-ice field –30°

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Figure 7 Sea-ice distribution at the EPILOG LGM timeslice compared to modern conditions. EPILOG LGM-WSI, maximum extent of winter sea-ice at the LGM (September, concentration > 15%); M-WSI, maximum extent of modern winter sea-ice (September, concentration > 15%); and M-SSI, maximum extent of modern summer sea-ice. Values indicate estimated winter (September) sea-ice concentration in percent derived with the modern analog technique and the generalized additive model. Large dots, concomitant occurrence of cold-water indicator Fragilariopsis obliquecostata (>1% of diatom assemblage) and summer sea-ice (February, concentration > 0%) interpreted to represent sporadic occurrence of EPILOG LGM-SSI (maximum extent of summer sea-ice at the LGM); medium dots, presence of winter sea-ice (September, concentration > 15%, diatom winter sea-ice indicators > 1%); small dots, no winter sea-ice (September, concentration < 15%, diatom winter sea-ice indicators <1%). Reproduced from Gersonde et al. (2005) Quaternary Science Reviews 24: 869-896, with permission from Elsevier BV.

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estimation of EPILOG LGM summer SSTs indicates a northward migration of the LGM Polar Front by 4 and 5–10 latitude in the Atlantic and Indian sectors, respectively, with the strongest cooling (4–6  C decrease) occurring in these sectors in the modern SubAntarctic Zone (Fig. 1). These are larger estimates than previously reported and Gersonde et al. (2005) conclude a similar northward shift in the LGM SubAntarctic Front in the Atlantic and Indian sectors but only a relatively minimal northward shift of the LGM Subtropical Front leading to compression of the SubAntarctic Zone and steepened thermal gradients. However, their data from the Pacific sector implies reduced cooling here and therefore possible nonuniform cooling (and asymmetric frontal movement) of the glacial Southern Ocean at the LGM. Antarctic Diatoms as Tracers of Bottom Water Marine planktonic diatoms endemic to the Southern Ocean can be transported out of Antarctic waters into the world’s oceans via Antarctic Bottom Water (AABW). This is possible where surface-dwelling diatoms are entrained into downwelling AABW during its formation in the Weddell and Ross seas and on the shelves around the Antarctic continent. Entrained diatoms subsequently settling out along the flow path become effective tracers of this deep current. This has been clearly demonstrated for modern times from diatoms in core-top sediments along the AABW flow path in the southwest and central Pacific and the southwest Atlantic. In these locations displaced endemic diatoms can comprise the majority of the sediment assemblage, while they are absent from core-top sediments located in the overlying water mass (e.g., the North Atlantic deep water, NADW). Endemic diatoms displaced by AABW have also been noted in surface and core sediments as far as 60  N. Significant abundances of displaced endemic diatoms have been found in late Quaternary (last ,200 ky) core sediments within the AABW flow path in the southwest Pacific and southwest Atlantic oceans. Along with the stable isotopic signal from biogenic carbonate, these assemblages help provide information on AABW production and flow history to these oceanic basins. This is of interest since AABW plays a key role in the climate system, being one of the main transporters of heat and salts around the world’s oceans. A higher abundance of displaced diatoms indicates higher AABW flow. For example, Jones and Johnson (1984) found that for the southwest Atlantic, displaced diatoms are more abundant during interglacials (higher AABW flow) than glacial

maxima (lower AABW flow). However, unlike stable isotopes, displaced endemic Antarctic diatoms provide an unequivocal tracer for bottom water provenance in the North Atlantic. Furthermore, in the southwest Pacific, downcore variations in the abundance of displaced open-ocean diatoms (e.g., Fragilariopsis kerguelensis; Fig. 3, Thalassiosira lentiginosa) versus sea-ice related diatoms (e.g., Fragilariopsis curta, F. cylindrus; Fig. 6) may indicate temporal switches in AABW source (open ocean versus shelf, respectively) through the late Quaternary. Diatom Biostratigraphy Planktonic marine diatoms make excellent biostratigraphic tools since they are widespread, easily extracted from the sediment, and have undergone several relatively rapid, stepwise, turnover (evolutionary) events during the Cenozoic, driven by climatic and tectonic changes. In the Southern Ocean they are the most useful microfossil group for biostratigraphy. The more recent Southern Ocean zonation scheme(s) provide relative and absolute age information and have been developed over many years from mainly offshore cores drilled by the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). Cenozoic zonation applies well to deep-sea Southern Ocean cores as well as those from the Antarctic shelf. Ramsey and Baldauf (1999) provide a review of the history of the development of the Southern Ocean biochronology. Although they are most useful for offshore sedimentary sections spanning longer timescales (e.g., Miocene to Recent), there are some well-known diatom events in the Quaternary that can be applied to younger core sediments and complement independent dating methods such as oxygen isotope stratigraphy. Due to latitudinal gradients of surface water parameters, however, the distribution of late Neogene and Quaternary marker diatoms is not the same north and south of the Polar Frontal Zone (Fig. 1). This means that the youngest part of the zonation does not work as well in the northern parts of the Southern Ocean as it does for sites south of the Polar Front and on the shelf. For this reason Gersonde and Ba´rcena (1998) and later Zielinski and Gersonde (2002) modified the late Pliocene and Pleistocene interval of the zonation for sites north and south of the Polar Frontal Zone using cores located mostly in the Atlantic sectors of the Southern Ocean (Fig. 8). Their modified northern Southern Ocean zonation critically includes the following biodatums of the subAntarctic (warmer water) marker species Hemidiscus karstenii, the abundance of which

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Diatom zones

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Figure 8 Compilation and correlation of the Plio-Pleistocene diatom zonation for the northern and southern part of the Southern Ocean. Both absolute and relative age information is provided: ages in (Ma); chronostratigraphy (epochs); and the geomagnetic polarity timescale (GPTS, polarity, and chrons). Reproduced from Zielinski and Gersonde (2002) Marine Micropaleontology 45: 225–268, with permission from Elsevier BV.

declines south of the Polar Frontal Zone: the First Abundant Occurrence Datum (FAOD) at 420 ky (lower MIS 11), and the Last Occurrence Datum (LOD) at 190 ky (upper marine isotope stage (MIS) 7) (Fig. 8). Their southern Southern Ocean zonation (applicable to south of the Polar Front) divides the Pleistocene differently based on biodatums of Antarctic (colder water) markers Rouxia constricta and R. leventerae: the LOD R. constricta at 280 ky (MIS 8), and the LOD R. leventerae at 130 ky (MIS 6) (Fig. 8). These new biodatums, along with their redefinition of the LOD Actinocyclus ingens biodatum improve the temporal resolution for southern Southern Ocean biostratigraphy by up to 100 ky.

Nonmarine Antarctic Waters: Freshwater Diatoms Less than 1% of the Antarctic continent is ice free and here diatoms can grow in lakes and streams as well as in terrestrial habitats. The majority of the lakes are situated on the coastal oases (e.g.,

McMurdo Dry Valleys, Vestfold Hills, Larsemann Hills, and Schirmacher Oasis), the ice-free areas of the Antarctic Peninsula (e.g., James Ross Island, King George Island, and Livingston Island), and the Antarctic Islands (e.g., Signy Island). Modern ecological and distributional data have been collected from several areas. The species diversity of nonmarine diatoms in Antarctica is low compared to Arctic regions, probably due to the physical isolation of the continent. The Antarctic Peninsula has a more diverse flora with a general trend of decreasing diversity southwards. Many Antarctic diatoms are classed as aerophilic but are also found in aquatic habitats subject to desiccation or freezing, others are found in association with terrestrial soils and mosses. Some taxa are widespread while others have a regional distribution and some are endemic. Holocene stratigraphic records have accumulated in many lakes, and due to the low diversity of Antarctic freshwaters and the absence of many other proxies conventionally used in temperate regions (e.g., pollen) diatoms have been extensively

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used for environmental reconstruction. Hodgson et al. (2004) provide a recent review of Antarctic paleolimnology in general and Spaulding and McKnight (1999) review the uses of diatoms in particular. Diatoms are good indicators of salinity, which in some lakes can be a proxy for climate, while in others is related to changes in sealevel. They have also been used as indicators of the changing extent of lake ice cover and lake depth, thus they are good indicators of climate and ecosystem change. Transfer functions have been developed for a range of environmental variables, for example, Jones and Juggins (1995) created a transfer function for nutrients (chlorophyll a) based on surface sediments obtained from Signy and Livingston Islands. This was applied to Holocene cores from Signy Island, and together with other proxy data showed a Holocene climate optimum of 3,800–1,300 14C yr BP when the productivity of the lakes was higher. Recent changes in the diatom flora related to nutrient enrichment from birds and seals were also observed. In the Vestfold and Larsemann Hills, Roberts and McMinn (1996), and Verleyen et al. (2003) have developed transfer functions for salinity. The water chemistry of lakes in these areas responds quickly to changes in the moisture balance, with increasing salinity and decreasing lake-water level during dry periods, with the opposite pattern in wet periods. Roberts et al. (2001) used a transfer function approach to obtain a high-resolution record of evaporation for the last 650 years from Ace Lake (Vestfold Hills) which was also highly correlated to the seasonal oxygen isotope signal preserved in an ice core from Law Dome, suggesting that the diatom-based reconstructions provide a robust paleoclimate signal. The McMurdo Dry Valleys located on the western coast of the Ross Sea are among the coldest and driest places on Earth. Lakes here are some of the harshest aquatic environments in the world. The lakes mostly have a perennial ice cover (2.8–6.0 m thick) over liquid water. The volume of these lakes has fluctuated greatly over the last 20,000 years and in recent times lake levels have been rising. Lake Hoare is a perennially ice-covered, closed-basin lake where diatoms form part of the benthic microbial mats, with characteristic species occurring in shallow-water, mid-depth, and deep-water communities. The lake has a unique sedimentary environment where due to lack of water circulation the sediments accumulate in situ and so record the history of a particular site rather than integrating events over the whole lake basin. Analysis of the sedimentary diatoms from Lake Hoare indicates that the rapid increase in lake level is a recent event. Diatoms have also been found in Antarctic ice cores such as Dome C and Taylor Dome; these are a mixture

of freshwater and marine taxa which have been blown in by long distance transport. At Taylor Dome, Kellogg and Kellogg (1996a and b) found that low abundances of diatoms occur during glacial intervals (MIS 3–5), and higher abundances occur during interstadials (MIS 5b, 5d, and 6). This is due to the conditions in the source areas. In glacials, the West Antarctic Ice Sheet covers most of the Ross Sea continental shelf and thus the nearest open water source for marine diatoms would be far offshore. The extent of ice-free areas in the dry valleys would also have been limited by lobes of grounded ice pushing into the valleys from the sea and by large ice-covered lakes greatly reducing possible sources for reworked marine diatoms.

Conclusion In this article we demonstrate how diatoms are a powerful paleoclimate tool in both marine and freshwater systems using fossil record examples from Antarctic waters. The nutrient-rich Southern Ocean is the world’s richest source of biogenic opal, resulting in a circum-Antarctic ‘siliceous ooze belt’. We summarize the latest information on diatom biogeography in surface sediments used to reconstruct past sea-surface temperatures (SSTs) and sea-ice patterns. We also discuss their use as bottom water tracers and for biostratigraphy. Records of non-marine diatoms have been found in lakes on the Antarctic continent and in ancient ice-cores. We review the uses of these Holocene records for climate and ecosystem change reconstructions.

Abbreviations AABW ACC ANDRILL

cal ky BP CCD CLIMAP

DD228 DSDP EPILOG FAOD GPTS

Antarctic Bottom Water Antarctic Circumpolar Current Antarctic Drilling Program (http:// andrill.org/ Also see this website for the history of Antarctic drilling including the Cape Roberts Project (CRP) and the New Zealand-led MSSTS and CIROS drilling projects and for related projects such as Shallow Drilling on the Antarctic Continental Margin (SHALDRILL)) calendar years before present calcium carbonate compensation depth CLIMAP (Climate and Environment Monitoring with GPS Atmospheric Profiling Diatom Database 228 Deep Sea Drilling Project Environment Processes of the Ice Age: Land, Ocean, Glaciers First Abundant Occurrence Datum geomagnetic polarity timescale

540 DIATOM RECORDS/Antarctic Waters HNLC IMAGES

IODP ka ky LGM LOD MIS NADW Ma ODP PF PFZ POOZ SAF SAZ SSI SST STF STZ SIZ WSI

high-nitrate, low-chlorophyll International Marine Past Global Changes Study (http://www.imagespages.org) Integrated Ocean Drilling Program thousand years ago thousand years Last Glacial Maximum Last Occurrence Datum marine isotope stage North Atlantic deep water millions of years ago Ocean Drilling Program Antarctic Polar Front Polar Frontal Zone Permanently Open Ocean Zone SubAntarctic Front SubAntarctic Zone maximum average summer sea-ice edge sea-surface temperatures Subtropical Front Subtropical Zone Sea Ice Zone maximum average winter sea-ice edge

See also: Diatom Introduction. Diatom Methods: Data Interpretation. Diatom Records: Marine Laminated Sequences. Glacial Climates: Thermohaline Circulation. Glaciations: Late Quaternary of Antarctica. Paleoceanography, Biological Proxies: Coccoliths; Marine Diatoms; Planktic Foraminifera; Radiolarians and Silicoflagellates. Paleoceanography, Physical and Chemical Proxies: Oxygen Isotope Stratigraphy of the Oceans. Paleoclimate: Time Scales of Climate Change. Quaternary Stratigraphy: Overview; Biostratigraphy; Chronostratigraphy. Sea Level Studies: Microfossil Reconstructions.

References Armand, L. K., and Leventer, A. (2003). Paleo sea ice distribution—reconstruction and paleoclimatic significance. In Sea Ice and Introduction to its Physics, Chemistry, Biology and Geology (D. N. Thomas and G. S. Diekm, Eds.), pp. 333– 372. Blackwell, Oxford. Armand, L. K., Crosta, X., Romero, O. E., and Pichon, J. J. (2005). The biogeography of major diatom taxa in Southern Ocean surface sediments: 1. Sea ice related species. Palaeogeography Palaeoclimatology Palaeoecology 223, 93–126. Barron, J. A. (2003). Planktonic marine diatom record of the past 18 m.y.: appearances and extinctions in the Pacific and Southern Oceans. Diatom Research 18, 203–224. Barron, J. A., and Baldauf, J. G. (1989). Tertiary cooling steps and palaeoproductivity as reflected by diatoms and biosiliceous sediments. In Productivity of the Ocean: Present and Past (W. H. Berger, V. S. Smetacek and G. Wefer, Eds.), pp. 341–354. John Wiley & Sons, New York. Barron, J. A., and Baldauf, J. G. (1995). Cenozoic marine diatom biostratigraphy and applications to paleoclimatology and paleoceanography. In Siliceous Microfossils (C. Blome, Ed.),

pp. 107–108. Paleontological Society, Short Courses in Paleontology, 8. Society of Economic Paleontologists and Mineralogists, Tulsa. Bohaty, S. M., Scherer, R. P., and Harwood, D. M. (1998). Quaternary diatom biostratigraphy and palaeoenvironments of the CRP-1 Drillcore, Ross Sea, Antarctica. Terra Antarctica 5, 431–453. Booth, J. D., and Burckle, L. H. (1976). Displaced Antarctic diatoms in the southwestern and central Pacific. Pacific Geology 11, 99–108. Burckle, L. H. (1984). Diatom distribution and paleoceanographic reconstruction in the Southern Ocean – present and Last Glacial Maximum. Marine Micropaleontology 9, 241–261. Burckle, L. H., and Cirilli, J. (1987). Origin of diatom ooze belt in the Southern Ocean: implications for late Quaternary paleoceanography. Micropaleontology 33, 82–86. Crosta, X., Pichon, J. J., and Burckle, L. H. (1998). Application of modern analog technique to marine Antarctic diatoms: Reconstruction of maximum sea-ice extent at the Last Glacial Maximum. Paleoceanography 13, 284–297. Crosta, X., Sturm, A., Armand, L., and Pichon, J. J. (2004). Late Quaternary sea ice history in the Indian sector of the Southern Ocean as recorded by diatom assemblages. Marine Micropaleontology 50, 209–223. Crosta, X., Romero, O. E., Armand, L. K., and Pichon, J. J. (2005). The biogeography of major diatom taxa in Southern Ocean surface sediments: 2. Open ocean related species. Palaeogeography Palaeoclimatology Palaeoecology 223, 66–92. Gersonde, R., and Ba´rcena, M. A. (1998). Revision of the upper Pliocene-Pleistocene diatom biostratigraphy for the northern belt of the Southern Ocean. Micropaleontology 44, 84–98. Gersonde, R., Crosta, X., Abelmann, A., and Armand, L. (2005). Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum—a circum-Antarctic view based on siliceous microfossil records. Quaternary Science Reviews 24, 869–896. Harwood, D. M., and Maruyama, T. (1992). Middle Eocene to Pleistocene diatom biostratigraphy of Southern Ocean sediments from the Kerguelen Plateau, Leg 120. In Proceedings of the Ocean Drilling Program, Scientific Results, 120 (S. W. Wise, Jr.,R. Schlich A. Palmer-Julson and E. Thomas, Eds.), pp. 683–734. Texas A&M University, Ocean Drilling Program, College Station, TX, USA. Hodgson, D. A., Doran, P., Roberts, D., and McMinn, A. (2004). Paleolimnological studies from the Antarctic and subAntarctic islands. In Long-Term Environmental Change in Arctic and Antarctic Lakes (R. Pienitz, M. S. V. Douglas and J. P. Smol, Eds.), pp. 419–474. Springer, Dordrecht. Jones, G. A., and Johnson, D. A. (1984). Displaced Antarctic diatoms in Vema Channel sediments: Late Pleistocene/Holocene fluctuations in AABW flow. Marine Geology 58, 165–186. Jones, V. J. (1996). The diversity, distribution and ecology of diatoms from Antarctic inland waters. Biodiversity and Conservation 5, 1433–1449. Jones, V. J., Hodgson, D. A., and Chepstow-Lusty, A. J. (2000). Palaeolimnological evidence for marked Holocene environmental changes on Signy Island, Antarctica. The Holocene 10, 43–60. Jones, V. J., and Juggins, S. (1995). The construction of a diatombased chlorophyll a transfer function and its application at three lakes on Signy Island (maritime Antarctic) subject to differing degrees of nutrient enrichment. Freshwater Biology 34, 433–445. Kellogg, D. E., and Kellogg, T. B. (1996a). Diatoms in South Pole ice: implications for eolian contamination of Sirius Group deposits. Geology 24, 118.

DIATOM RECORDS/Freshwater Laminated Sequences Kellogg, D. E., and Kellogg, T. B. (1996b). Glacial/interglacial variations in the flux of atmospherically transported diatoms in Taylor Dome ice core. Antarctic Journal of the United States Review. Lozano, J. A., and Hays, J. D. (1976). Relationship of radiolarian assemblages to sediment types and physical oceanography in the Atlantic and Western Indian Ocean Sectors of the Antarctic Ocean. In Investigation of late Quaternary Paleoceanography and Paleoclimatology (R. M. Cline and J. Hays, Eds.), pp. 303–336. Geological Society of America, Boulder, CO, USA. Pichon, J. J., Labeyrie, L. D., Bareille, G., et al. (1992). Surface water temperature changes in the high latitudes of the Southern Hemisphere over the last glacial–interglacial cycle. Paleoceanography 7, 289–318. Ramsay, A. T. S., and Baldauf, J. G. (1999). A Reassessment of the Southern Ocean Biochronology, p. 122. Geological Society, London. Roberts, D., and McMinn, A. (1996). Relationships between surface sediment diatom assemblages and water chemistry gradients in saline lakes of the Vestfold Hills, Antarctica. Antarctic Science 8, 331–341. Roberts, D., van Ommen, T. D., McMinn, A., Morgan, V., and Roberts, J. L. (2001). Late Holocene East Antarctic climate tends from ice core and lake sediment proxies. The Holocene 11, 117–120. Romero, O. E., Armand, L. K., Crosta, X., and Pichon, J. J. (2005). The biogeography of major diatom taxa in Southern Ocean surface sediments: 3. Tropical/subtropical species. Palaeogeography Palaeoclimatology Palaeoecology 223, 49–65. Scherer, R. P. (1991). Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic West Antarctic Ice Sheet history. Palaeogeography Palaeoclimatology Palaeoecology 90, 395–412. Sjunneskog, C., and Scherer, R. P. (2005). Mixed diatom assemblages in glacigenic sediment from the Central Ross Sea, Antarctica. Palaeogeography Palaeoclimatology Palaeoecology 218, 287–300. Spaulding, S. A., and McKnight, D. M. (1999). Diatoms as indicators of environmental change in Antarctic freshwaters. In The Diatoms: Applications for the Environmental and Earth Sciences (E. Stoermer and J. P. Smol, Eds.), pp. 245–263. Cambridge University Press, Cambridge. Spaulding, S. A., McKnight, D. M., Stoermer, E. F., and Doran, P. T. (1997). Diatoms in sediments of perennially ice-covered Lake Hoare, and implications for interpreting lake history in the McMurdo Dry Valleys of Antarctica. Journal of Paleolimnology 17, 403–420. Stickley, C. E., Carter, L., McCave, I. N., and Weaver, P. P. E. (2001). Lower circumpolar deep water flow through the SW Pacific Gateway for the last 190 ky: evidence from Antarctic diatoms. In The Oceans and Rapid Climate Change, Past, Present and Future (B. J. Haupt and M. A. Maslin, Eds.), pp. 101–116. Geophysical Monograph Series, 126, Washington. Stickley, C. E., Pike, J., Leventer, A., et al. (2005). Deglacial ocean and climate seasonality in laminated diatom sediments, Mac. Robertson Shelf, Antarctica. Palaeogeography Palaeoclimatology, Palaeoecology 227, 290–310. van de Vijver, B., and Beyens, L. (1999). Biogeography and ecology of freshwater diatoms in subAntarctica: a review. Journal of Biogeography 26, 993–100. Verleyen, E., Hodgson, D. A., Vyverman, W., et al. (2003). Modelling diatom responses to climate induced fluctuations in the moisture balance in continental Antarctic lakes. Journal of Paleolimnology 30, 195–215. Winter, D. M., and Harwood, D. M. (1997). Integrated diatom biostratigraphy of Late Neogene drillholes in Southern Victoria

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Land and correlation to Southern Ocean records. In The Antarctic Region: Geological Evolution and Processes (C. A. Ricci, Ed.), pp. 985–992. Terra Antarctica Publication, Sienna. Zielinski, U., and Gersonde, R. (1997). Diatom distribution in Southern Ocean surface sediments (Atlantic Sector): implications for paleoenvironmental reconstructions. Palaeogeography Palaeoclimatology Palaeoecology 129, 213–250. Zielinski, U., and Gersonde, R. (2002). Plio-Pleistocene diatom biostratigraphy from ODP Leg 177, Atlantic Sector of the Southern Ocean. Marine Micropaleontology 45, 225–268. Zielinski, U., Gersonde, R., Sieger, R., and Fu¨tterer, D. (1998). Quaternary surface water temperature estimations: calibration of a diatom transfer function for the Southern Ocean. Paleoceanography 13, 365–384.

Freshwater Laminated Sequences H Simola, University of Joensuu, Joensuu, Finland ª 2007 Elsevier B.V. All rights reserved.

Introduction Varves and Diatoms—Ideal Objects for Paleoecological Studies Varved or annually laminated sediments contain high-precision records of sedimentation with yearly and sometimes even seasonal resolution (Lamoureux, 2001). Varves, especially in unconsolidated sediments, may be difficult to discern or may easily be disturbed by coring, so they are best sampled by freeze-coring (Glew et al., 2001). Varve formation is a consequence of low-energy sedimentation and an absence of bioturbation, and it can therefore also indicate good preservation of various constituents of the sediment. A particular benefit of varves is that they provide a precise chronology for the sediment sequence. The chronology may be floating or absolute (calendar), depending on whether it is possible to fix the series to the present day or to an historical event, and it may be more or less accurate, depending on the regularity and ease of identification of the annual cycles. Diatoms are often abundant and are usually well preserved in lake sediments (Battarbee et al., 2001). Sedimentary diatom analysis is a versatile tool for tracking environmental history (Charles et al., 1994; Flower et al., 1997; Smol, 2004). Diatoms in varved sediments often provide environmental records of very high quality, although the relative rarity of varved sediments sets limits to their routine use as environmental archives. In many ways, however, it is possible to improve the interpretation of