Record of the early Holocene warming in a laminated sediment core from Cape Hallett Bay (Northern Victoria Land, Antarctica)

Global and Planetary Change 45 (2005) 193 – 206 www.elsevier.com/locate/gloplacha

Record of the early Holocene warming in a laminated sediment core from Cape Hallett Bay (Northern Victoria Land, Antarctica) Furio Finocchiaroa,*, Leonardo Langoneb, Ester Colizzaa, Giorgio Fontolana, Federico Gigliob, Eva Tuzzia a

Dipartimento Scienze Geologiche, Ambientali e Marine, Universita` di Trieste, v. E. Weiss, 2-34127 Trieste, Italy b ISMAR-CNR, Sezione Geologia Marina, Via Gobetti, 101-40129 Bologna, Italy Received 16 September 2003; accepted 28 September 2004

Abstract This paper presents an integrated multiproxy approach study (sedimentological, geochemical, preliminary smear-slides diatom assemblages, and 14C ages analyses) performed on a sediment core collected in Cape Hallett Bay (Ross Sea, Antarctica). Sediments record the early Holocene rapid climate changes: buried varved diatomaceous ooze on the base of core (N9.5–9.4 ka BP) are linked to the early Holocene warming and open marine conditions. From 9.4 ka BP, the climate starts to cool (massive mud). From 8.0 to 7.8 ka BP, sandy mud sediment suggests a rapid landward recession of the local/regional glaciers, with relevant underflow inputs, together with the onset of seasonal sea-ice formation. The ages and the characteristics of the youngest sediments are related to the changed oceanographic conditions linked to the retreat of the calving front of the Ross Ice Shelf. D 2004 Elsevier B.V. All rights reserved. Keywords: Paleoenvironmental reconstruction; Laminated sediments; Early Holocene; Ross Sea; Antarctica

1. Introduction Recently, research on paleoclimatic reconstruction in Antarctic areas has focused on short-term climatic changes during the Holocene. This period is crucial for understanding the present climatic system and

* Corresponding author. Tel.: +39 040 5582025; fax: +39 040 5582048. E-mail address: [email protected] (F. Finocchiaro). 0921-8181/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2004.09.003

predicting future global changes. Review of ice-core data has identified two climatic optima during the Holocene, the first between 11.5 and 9 ka BP, and a second between 6 and 3 ka BP in the Eastern Antarctic sector and between 7 and 5 ka BP for the Ross Sea area (Masson et al., 2000). According to Cias et al. (1992), the period of maximum Holocene warmth in East Antarctica ice cores is between 10 and 7.5 ka BP; Siegert (2001) indicates a warm peak at 9.4 ka BP in Southern Hemisphere. Timing of the early climatic optimum coincides with a peak in

194

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

abundance of foraminifera in South Atlantic sediments (Hodell et al., 2001). Baroni and Orombelli (1994) reported a mid-Holocene warm phase on the basis of penguin rookery occupation along Victoria Land coast. The study of this restricted time interval needs high-resolution geological records, and for marine sediments, continuous and expanded sedimentary successions. Marine successions characterized by high-sedimentation rates in late Quaternary shelf sediments around Antarctica have been found mainly in embayments and fjords along the Antarctic Peninsula coast. There, sedimentation rates are

higher than 100 cm ka 1 (Lallemand Fjord, Domack et al., 1995; Palmer Deep, Leventer et al., 1996; Domack et al., 2001) with maximum values of 1500 cm ka 1 (Brialmont Cove; Domack and McClennen, 1996). Recently, another site of very high accumulation rate in laminated diatomaceous ooze (290 cm ka 1) was found in George V Land continental shelf (Harris et al., 2001). Marine sediments in this sector also record two climatic optima: the first in the early Holocene, the second one in the mid-Holocene (Pudsey et al., 1994; Leventer et al., 1996; Domack et al., 2001; Taylor et al., 2001; Taylor and McMinn, 2001; Presti et al., 2003; Goodridge, 1999–2000).

Fig. 1. Location of core ANTA02-CH41. Bathymetric data and land morphology from map SS 58-60/2 (Cape Hallett), original scale 1:250,000, edited by U.S. Geological Survey.

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Concerning the mid-Holocene optimum, authors are not in agreement about exact timing and period of this phase. In the Ross Sea, information on Holocene climatic changes based on the sediment record is very scarce. Offshore basins of the Ross Sea continental shelf show generally low sedimentation rates: the highest values (ca. 20–24 cm ka 1) characterize modern biogenic mud in the deepest and central part of the Joides Basin (Frignani et al., 1998; Finocchiaro et al., 2000). Only inside Granite Harbor, where the thickness of diatomaceous ooze can be at least 10 m (Domack et al., 1999), the sedimentation rate is very high (up to 250 cm ka 1, DeMaster et al., 1996), reinforcing the observation that bays and fjords have excellent potential to preserve high-resolution sedimentary records. Many authors recognized that the retreat of the grounding line in the Ross Sea began about 14–13 ka BP (Stuiver et al., 1981; Denton et al., 1989; Licht et al., 1996; Brambati et al., 1997). The general ice withdrawal was affected by minor climatic fluctuations (Steig et al., 1998; Brambati et al., 1997; Orsini et al., 2003). Recently, detailed studies on diatom assemblages have identified a warmer period between 6 and 3 ka (Cunningham et al., 1999), while high resolution in late Holocene paleoclimatic reconstruction has been obtained from Granite Harbor sediments (Leventer et al., 1993). Evidence of the early Holocene climatic warm phase is herein reported based on results from a gravity core (ANTA02-CH41) collected in Cape Hallett Bay (Lat. 72817.49VS–Long. 170809.05VE; water depth: 416 m) during the 2001–2002 austral summer (Fig. 1). The early Holocene time interval is well documented due to very high sediment accumulation rates at the coring site. A succession of climatic events was tentatively reconstructed based on paleoenvironmental inferences.

2. Material and methods Cape Hallett is located along the northern Victoria Land coast, about 110 km north of Coulman Island (Fig. 1). The sediment core was collected at the entrance of the Edisto Inlet, along the conjunction between Cape Hallett and Cape Christie. Edisto Inlet

195

is a small bay, about 15 km long and 4 km wide, deeper than 500 m and separated by a sill from the larger Moubray Bay (USGS, 1968). The Edisto Glacier is small and flows into the inner part of the bay, whereas a saddle at only 800 m above sea level separates (a few kilometers southward) the bay from the terminal section of the Tucker Glacier. The core site is seaward (north) of the sill, and a SBP 3.5 kHz seismic reflection profile shows parallel and subparallel reflectors of a stratified sequence about 8 m thick (Bussi et al., 2003). The 408-cm-long sediment core was collected from the site using a 2.3ton gravity corer. The core was scanned for magnetic susceptibility, split, X-radiographed, described, and sub-sampled. The upper 3 m of core are massive and were sampled in 1-cm-thick slices taken at 10-cm intervals. Below 3 m, the sediment has alternating dark and light layers. Forty-two layers were sampled along this interval. Samples were dried at 60 8C and then slightly disaggregated for the following analyses. Porosity was calculated based on water content according to Berner (1971) and assuming a mineral density of 2.55 g cm 3. Particle size was determined after treatment with H2O2 and wet sieving at 63 Am. Sandy samples were then analyzed by a MacroGranometer sedimentation balance and muddy samples by a Sedigraph 5100 ET (Micromeritics). A Coulter Multisizer (100-Am orifice tube) was used for very small muddy samples from the laminated section. Organic carbon content of each sample was determined using a FISONS NA2000 Elemental Analyzer (EA) after removal of the carbonate fraction by adding HCl 1.5 N. The errors associated with determinations are around 1%. Biogenic silica content was determined through a progressive dissolution method (DeMaster, 1981), followed by colorimetric analysis. We used NaOH 0.5 M as an extractant in view of the significant concentrations of biogenic silica usually found in Antarctic samples (DeMaster, 1981). Forty-three smear slide observations provided preliminary information on diatom assemblages. The chronology of the sediment core was defined by means of 7 AMS 14C ages determined on the bulk organic fraction at Geochron Laboratories (Cambridge, MA, USA).

196

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

3. Results Preliminary data of core ANTA02-CH41 were reported by Finocchiaro et al. (2003). Two main lithological units can be recognized in the core (Fig. 2). In the lower unit (Unit A, 408–207 cm), two subunits (A1 and A2) can be distinguished. Sub-Unit A1 (408–307 cm) is characterized by a rhythmic sequence of parallel light and dark mud laminae, from 0.5 to 19.5 mm thick (Fig. 3). Only 10% of laminae has a thickness between 10.5 and 19.5 mm, and we report them as dthick laminaeT, following the terminology of Reineck and Singh (1973). Colors of laminae vary from black-gray to several shades of olive (dark olive to pale olive). In general, contacts between light laminae and overlying dark laminae are gradational, whereas the contacts from dark to light laminae are sharp. From preliminary observations of smear slides, light laminae are almost entirely composed of diatom frustules, whereas volcaniclastic silt is a subordinate component of dark laminae. The compositional difference between light and dark laminae is also testified by the slightly higher values of organic C

(0.85 vs. 0.73 wt.% for light and dark laminae, respectively), but their biogenic silica values are similar (34.1 vs. 33.3 wt.%). The particle size of laminae of different color was measured: dark laminae are poorly sorted with a coarse-grained tail, whereas olive laminae have the same modal diameter (12–15 Am), although it is more leptokurtic. Pale olive laminae have better sorting and a finer mode (6–7 Am; Fig. 4). The different particle size distributions of light laminae are probably related to different diatom assemblages. Some light laminae (pale olive) show a bfluffyQ texture, very similar to the description of the bcottonyQ layer found in cores from Granite Harbor and MacRobertson Bay (Leventer et al., 1993; Taylor and McMinn, 2001). Number and thickness of laminae were measured. Two hundred and sixteen laminae have been defined, well recognizable by both direct visualization and different X-ray beam attenuation in radiographs. Total thickness of 108 pairs of laminae (light+dark lamina couplets) varies from 3 to 23 mm (mean value of 8.9 mm) with frequent oscillations, but the trend is to thin upward (Fig. 5a). The light laminae are generally thicker than those dark in color. The

Fig. 2. Core ANTA02-CH41: Lithostratigraphy and unit identification; mass magnetic susceptibility (from Finocchiaro et al., 2003); water content (%) on wet weight; concentration of organic carbon and biogenic silica (wt.%).

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Fig. 3. Photograph (left) and positive X-ray (right) of the parallel laminated interval Sub-Unit A1. Scale in centimeters, from the core top.

197

ratio between dark lamina and total thickness of light–dark pair tends to increase upwards, due to reducing thickness of the light laminae, whereas the dark laminae remain almost constant in thickness (Fig. 5b). Sub-Unit A2 (307–207 cm) is a massive dark olive-grey mud with wavy and irregular laminae concentrated between 253 and 207 cm. In comparison with Sub-Unit A1, organic carbon and biogenic silica decrease (mean values 0.77 and 18 wt.%, respectively), whereas the sand content increases. Grain size of massive sediment shows a prevalence of silt, with a modal diameter between coarse silt and very fine sand (Fig. 4). The upper unit (Unit B, 207–0 cm) is characterized by sand (Figs. 2 and 4) with dispersed gravel clasts as dropstones. Two subunits can be distinguished in this section: the lower subunit (Sub-Unit B1, 70–207 cm) is a dark olive-grey muddy sand with a sand content varying between 33% and 85%. Organic carbon and biogenic silica contents of SubUnit B1 are the highest of Unit B (mean values, 0.37 and 5 wt.%, respectively). The Sub-Unit B2 (0–70 cm) is very dark grey, slightly muddy sand, with sand content varying from 68% to 70%, and the lowest values of organic carbon (0.24 wt.%) and biogenic silica (2%). Preliminary diatom analyses show that Corethron pennatum (=C. criophilum: Crawford et al., 1998), Fragilariopsis curta, together with Chaetoceros resting spores account for up to 96% of the diatom assemblages in the whole core. C. pennatum occurs only in the laminated levels (Sub-Unit A1 and only partially in Sub-Unit A2), whereas F. curta and Chaetoceros r.s. are found in different percentages throughout the core. F. curta becomes dominant in the upper, coarse-grained Unit B. Finally, we tried to determine if the total thickness of the light+ dark pair and dark/light lamina ratio have cyclical trends through time, considering each couplet as a 1-year deposit. Although the autocorrelation and Fourier analyses did not give statistically significant results, total thickness data and dark/light thickness ratios show 11- and 13-year major periodicity cycles, respectively. Standard Z statistics was applied on autocorrelation coefficients for the lag =11 and 13 years, giving Z t (11 years) = 1.60 and Z t (13

198

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Fig. 4. Particle-size data: (a) Sub-Unit A1: average frequency curves of dark, pale olive and dark olive laminae measured by Coulter Multisizer; (b,c,d) from Sub-Units A2, B1, and B2, respectively, frequency curves of samples, measured by sedimentation balance and Sedigraph.

years)=1.85, slightly lower than 95% probability limit (Z t=1.96).

4. Age model and sediment accumulation rates Due to the absence of carbonate, it was necessary to date the bulk organic matter in the sediment. Seven radiocarbon dates of bulk organic matter (Table 1) were used to set the chronological constraint (Fig. 6) and estimate sedimentation rates (Fig. 7). Radiocarbon ages from Antarctic material must be interpreted with great caution because of the uncertainty of several factors, such as: reservoir effect, vital effect among different organisms, and other minor effects (Domack et al., 1999). In addition, sediments may be variably contaminated with reworked carbon (Harris et al., 1996). In this regard, diatomaceous mud and ooze units provide the most accurate determinations of radiocarbon age, because the material has the highest concentration of autochthonous organic matter of all the lithofacies. Ages at or prior to the last glacial maximum (LGM) are interpreted as mixed (Domack et al., 1999). Most authors use a reservoir correction

ranging between 1.2 and 1.5 ka for the Ross Sea region (Stuiver et al., 1981; Licht and Andrews, 1997; Berkman, 1997; Licht et al., 1996, 1999) and for other Antarctic shelf areas (Pudsey et al., 1994; Shipp and Anderson, 1994; Gingele et al., 1997). Nevertheless, the problems that plague radiocarbon dating and the high spatial variability of surface sediment ages in the Antarctic marine system mean that it is unlikely that a reliable absolute age for a single-dated horizon within a sediment core can be obtained (Domack et al., 1999). Adjustments could be made by subtracting the age of the organic matter at the sediment–water interface for each core, but one needs to be certain that the sediment–water interface has been recovered and sediment mixing also must be evaluated (Domack et al., 1999). By comparing the magnetic susceptibility profiles with a companion box-core collected in the same site (data not shown), the uppermost 2 cm of the sedimentary succession of core ANTA02-CH41 seems to have been lost during coring operations. If a sediment constant accumulation rate of 12.6 cm ka 1 is assumed between the levels 0–1 and 34–35 cm, then interpolated age at the sediment–water interface is 1630

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

199

Fig. 5. Thickness variation of laminae of Sub-Unit A1 (307–408 cm) relative to: (a) 108 dark+light laminae couplets; (b) dark laminae, and (c) light laminae.

years, which is very near to the most commonly used value for reservoir corrections. Radiocarbon results were consequently corrected by subtracting the calculated age of the organic carbon at the core top and the bias due to the top loss during core

sampling. According to the corrected data, the studied core spans from early Holocene to the Recent. There is a progression of ages down core, with the exception of three dates measured in Sub-Unit A1, which are not completely consistent with values above. Only part of

200

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Table 1 AMS 14C ages of ANTA91-CH41 core, on bulk organic matter Cruise

Core

Level

Code

14

F(1r)

ANTA02 ANTA02 ANTA02 ANTA02 ANTA02 ANTA02 ANTA02

CH41 CH41 CH41 CH41 CH41 CH41 CH41

00–01 34–35 70–71 232 300 369 402

GX-29188 GX-30574 GX-29991 GX-29189 GX-29190 CX-29992 GX-29191

1790 4490 9400 9970 10,920 9130 10,070

40 50 40 50 50 40 50

the difference can be also accounted for by reproducibility variability (Andrews et al., 1997, 1999). More likely, the inversion could be due to the release of 14C depleted glacial meltwater, a variable reservoir effect through time (van Beek et al., 2002) or contamination during subsampling. Alternatively, if the lamina couplets are an annual record of surface productivity, the Sub-Unit A1 comprises the sediment accumulating in a very short time interval (about one century). To overcome the problem of the lower three dates and to calculate sediment accumulation rates, an age model was developed. For the uppermost 300 cm, ages of samples were estimated assuming a constant sedimentation rate

Fig. 6. Age-depth model of the core ANTA02-CH41. Conventional 14 C dates were corrected for a reservoir age of 1630 years. The age model was developed using an integrated approach: in the uppermost part of the core (0–300 cm), ages were calculated based on a constant sediment accumulation rate for each lithological unit; below 300 cm, an assumption was made that each couplet was a varve.

C age (year BP)

d 13C (x vs. PDB) 25.8 25.5 28.6 27.3 26.7 31.9 28.4

within each lithological unit. In detail, in the Sub-Unit A2, a sediment accumulation rate of 71.6 cm ka 1 was calculated based on corrected 14C dates measured at 232 and 300 cm. Then, the ages at the boundaries of Sub-Unit A2 (207 and 307 cm) were extrapolated, assuming for the whole subunit the same sedimentation rate measured between 232 and 300 cm. To calculate the sediment accumulation rate of SubUnit B1, the measured and calculated ages of 70.5 and 207 cm depth, respectively, were used. Downcore, for Sub-Unit A1 (307–408 cm), we assumed an annual varved-like sedimentation (see previous discussion). By assuming this age model (Fig. 6), we calculated linear sediment accumulation rates and fluxes of bulk mass, biogenic (organic carbon and biogenic silica) and lithic components (Fig. 7). In early Holocene, represented by Sub-Unit A1, the linear accumulation rate is very high (926 cm ka 1). Sediment accumulation rates decrease (71.6 cm ka 1) in Sub-Unit A2, increase again in Sub-Unit B1 (615.7 cm ka 1), and then becomes low (9.3 cm ka 1) during the last 7.8 ka 1 in Sub-Unit B2. A quite different picture is obtained when we observe the variation of mass accumulation rates with time (Fig. 7b) due to the different porosity and bulk dry density between the sediment above and below 300 cm. In fact, the mass accumulation rate shows the highest values in Unit B1 (8.0–7.8 ka BP) and the lowest (12.2 g cm 2 ka 1) in the last 7.8 ka BP (Unit B2). However, the strong variability of sediment composition that characterizes the time interval between 9.5 and 7.8 ka results in a partition in three time intervals: the oldest, equivalent to the laminated section (9.5– 9.4 ka), is characterized by the highest biogenic fluxes of both organic carbon and biogenic silica (Fig 7c,d), whereas the youngest period (8.0–7.8 ka) shows the highest fluxes of the lithogenic fraction

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

201

Fig. 7. (a) Linear (dashed line) and mass (solid line) accumulation rate vs. sediment depth (cm); (b) mass accumulation rate (g cm 2 ka 1) vs. time (ka). Temporal variations of the mass accumulation rate (MAR) of biogenic silica (BSi), Corg and lithic fraction were plotted with more detail between 10 and 7.5 ka BP, in (c), (d), and (e), respectively.

(Fig. 7e). The accumulation of both lithogenic and biogenic components (Fig. 7c–e) are relatively low in the intermediate time interval (9.4–8.0 ka).

5. Discussion and conclusions 5.1. Laminated sediments (Sub-Unit A1) The most common stratigraphic succession on the Ross Sea shelf has a basal diamicton unit of glacial origin at its base (Anderson, 1999; Domack et al., 1999; Brambati et al., 2002). Overlying facies are composed of glacimarine silt and sandy intervals. The uppermost portion of most cores collected in the Ross Sea is a diatomaceous ooze (Dunbar et al., 1985; Licht et al., 1996; Frignani et al., 1998; Langone et al., 1998; Domack et al., 1999) interpreted as having been deposited in seasonally open water conditions. The presence of fine diatomaceous mud buried under about 2 m of sandy sediment is peculiar of core

ANTA02-CH41, and indicates open marine conditions during early Holocene. Despite the fact that buried diatomaceous sediment was recently recognized by Colizza et al. (2003) south of the Drygalski basin, this stratigraphic sequence is quite rare in the Ross Sea. Fine-grained laminated sediments are widely reported in Late Quaternary Antarctic sequences. For example, highly laminated sediments characterize the drift area of the Pacific margin of the Antarctic Peninsula (Pudsey and Camerlenghi, 1998; Lucchi et al., 2002), Palmer Deep and Gerlache Strait (Leventer et al., 2002; Goodridge, 1999–2000), the MacRobertson Shelf (Harris, 2000), the George V continental shelf (Wilkes Land Margin: Domack and Anderson, 1983; Domack, 1988; Brancolini and Harris, 2000; Presti et al., 2003), and outer slope areas of the Ross Sea embayment (Bonaccorsi et al., 2000). In the inner shelf of the Ross Sea, laminated sediments are reported by Nishimura et al. (1998) (north of Ross Island), Leventer et al. (1993) (Granite

202

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Harbour) and Colizza et al. (2003) (Wood Bay area). In general, the preservation of parallel lamination in marine sediment is considered a proxy for anoxic conditions, that exclude the presence of a benthic community, or a very high sedimentation rate in areas characterized by upwelling or by favorable oceanographic conditions (Grigorov et al., 2002; Cofaigh and Dowdswell, 2001). 5.2. Facies interpretation and paleoenvironmental reconstruction Based on physical, geochemical, and biological proxies, and using the age-depth model, a sequence of depositional events driven by paleoclimate and paleoenvironmental changes was tentatively reconstructed. Particular attention was devoted to the early Holocene, a time interval well preserved in sediments of Cape Hallett Bay. 5.2.1. Sub-Unit A1 (N9.5–9.4 ka BP) The high accumulation rate and high organic carbon and biogenic silica contents in the Unit A (about 2 m thick) of core ANTA02-CH41 suggest a period of high biological productivity. Based on the modern assemblage distribution (Cunningham and Leventer, 1998), the Corethron occurrence is associated with a well-stratified water column linked to weak wind, thermal warming and/or seaice meltwater (Leventer et al., 1996). The olive laminae are mainly composed by Corethron and Chaetoceros r.s. Few pale-olive, fluffy, thick laminae contain a nearly monospecific Corethron assemblage, which produces pulses of rapidly sinking algal flocs during early spring blooms. In the following dark lamina, the contribution of fine glacial debris mainly from overflow plumes increases through summer. Thus, the pair of laminae depicts a varve-like sedimentation with a seasonal alternation of productivity events (light laminae) and deposition of terrigenous debris (dark laminae). All these proxies, together with the age model, constrain the deposition of the lowest sedimentary unit to the early Holocene warming. Leventer et al. (1993) documented a layer of Corethron ooze that was related to deposition during the Medieval Warm Period.

5.2.2. Sub-Unit A2 (9.4–8.0 ka BP) Climatic conditions were not constant during the time of fine-grained sediment accumulation (Unit A): the reduction of light lamina thickness in the upper part of Sub-Unit A1 implies a general decrease in productivity over the period, thus suggesting a progressive climate cooling. The massive mud of Sub-Unit A2 further indicates the end of the optimal climatic conditions of the early Holocene. However, the shifting is punctuated by several oscillations marked by an alternation between massive and irregularly laminated levels. In Sub-Unit A2, both biogenic silica and sand contents show intermediate values from the laminated interval and the Sub-Unit B1, above. The low lithic mass accumulation rate (about one third of the Sub-Unit A1), together with the significant increase of F. curta and Chaetoceros in diatom assemblages, imply a climate cooling which is likely to be associated to a more persistent sea-ice covering. Subsequently, during a shorter and cooler summer, a low amount of meltwater prevents the formation of stratification along the water column; in the same manner, fine sediment release from overflow plumes is sensibly lower than during the Holocene warming. The sedimentary signature is therefore given by the superimposition of fine detrital material settled during summer, arranged in amalgamated and structureless (by bioturbation) layers, only occasionally interrupted by laminated biosiliceous mud, typical of warmer phases. 5.2.3. Sub-Unit B1 (8.0–7.8 ka BP) The abrupt change at level 207 cm depth (corresponding to 8.0 ka BP) can be related to a dramatic environmental change. All paleoproductivity markers decrease, particle-size becomes coarser and consequently, the mass accumulation rate of lithics exceeds the biogenics. Corethron disappears, whereas F. curta becomes the prevailing diatom species. High percentages of F. curta have been associated with marginal sea-ice zones (Leventer et al., 1996; Cunningham et al., 1999). The high accumulation of the terrigenous sediment could document the rapid landward recession of the local and/or regional glaciers and the onset of seasonal seaice formation.

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

In the Ross Sea region, the ice sheet retreated by 11 ka BP (Domack et al., 1999). A possible trigger of the fast retreat of local glaciers could be linked to a relative sea level rise following the regional retreat of the East Antarctica Ice Sheet (EAIS). The rapid EAIS retreat and related sea level rise may well have forced instability of the local glacier grounding line pinned on the morphological high at the entrance of Edisto Inlet (Fig. 1). Consequently, the glacier grounding line rapidly migrated to inner positions of the inlet. In some cores from the continental shelf of the Ross Sea, the transition from glacial (diamicton) to glacimarine sediment and/or siliceous mud is marked by a sorted muddy sand layer. The occurrence of this layer was first pointed out by Kellogg et al. (1979) and then described as a bgranulated faciesQ by Domack et al. (1999). This coarse grained sediment has been interpreted as a meltwater facies, related to decoupling and lift-off of a recessional melting line of the ice sheet. The depositional model interprets the granulated facies as resulting from melting of basal ice near the grounding zone, linked to strong bottom tidal currents that have winnowed out the fine fractions, thus increasing sand percentages and sorting (Domack and Harris, 1998). The sandy level recognized at the base of this subunit, may be the equivalent of the granulated facies, representing the lift-off of local glaciers in the Cape Hallett area. Despite the moderate increase of the mud content, the remnant upper part of the Sub-Unit B1 features grain size distribution with marked sorting characteristics (Fig. 4). These data, together with the high sedimentary rates and increase in ice-rafted debris (IRD), suggest a progressive landward recession of the glacier front, which caused an abundant release of dense underflow plumes. A new circulation pattern inside the fjord, supported by high meltwater fluxes and sea-level rise, is likely to be the main triggering factor for bottom currents able to winnow and transport sediments toward the deepest part of the bay, far from the supply zone of coarse debris (glacier margin subaqueous fan; Boulton, 1990). 5.2.4. Sub-Unit B2 Proxies at the base of Sub-Unit B1 (140–207 cm) and in Sub-Unit B2 (0–70 cm) are quite similar, suggesting that similar processes affected sedimentation processes, but the sedimentation rate dramatically

203

decreased to values typical of offshore Holocene sediments of the Ross Sea: 9.1 cm ka 1 (Brambati et al., 1997; Frignani et al., 1998) during the last 7.9 ka. This difference may indicate the retreat of the calving front of the Ross Ice Shelf over the study site at about 8.0 ka BP and the reduction of the glacial debris supplied only by small glaciers flowing into the Edisto Inlet. Retreat of the ice sheet from most of the Western Ross Sea area had as a consequence the onset of the Ross Sea and Terra Nova polynyas and the formation of High Salinity Shelf Water (HSSW), the densest waters of the Ross Sea continental shelf. The HSSW feeds bottom currents flowing out the Ross Sea along the coast of the Victoria Land. Establishment of these conditions also determined a more pronounced intrusion of the Circumpolar Deep Water onto the shelf (Denton et al., 1989) that, in turn, contributed to raise to present-day levels the diatom productivity that characterizes the southwestern Ross Sea and the JOIDES basin. The uppermost lithofacies (0–70 cm) of core ANTA02-CH41 is a common feature in the northwestern Ross Sea. In fact, coarse sediment was also observed in surficial sediments collected on the continental shelf between Coulman Island and Cape Adare (Melis et al., 2002), supporting the hypothesis of a similar origin. 5.3. Concluding remarks This study sheds new light on the sedimentary record of rapid climatic changes affecting the early Holocene in the Antarctic region. An integrated multiproxy approach has allowed the documentation of: ! A possible varved biogenic ooze, which implies a recovery of the record of the early Holocene warming in sediments of the Ross Sea. Sedimentological and geochemical parameters, together with floral assemblage observations, indicate this time period (N9.5–9.4 ka BP) as being characterized by weak winds and thermal warming of the strongly stratified upper water column. Biological productivity was highly enhanced with substantial early spring algal blooms, which produced episodic events of sediment deposition of rapidly

204

!

!

!

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

sinking biogenic material. During the following summers, thermal warming released a peak of fine glacial debris. The result was the accumulation of an annual couplet of alternating light and dark laminae. Starting at 9.4 ka BP, the climate cooled. The algal assemblages show more stressed environmental conditions, and also, glacial melting diminished. The shift to these conditions is quite well marked, although interrupted by several short-lived warm oscillations. From 8.0 to 7.8 ka BP, the sediment core records a high flux of terrigenous material, coarse and wellsorted at the beginning and finer at the end of this time interval, which was interpreted as the product of a rapidly receding glacier. The diatom assemblages suggest that seasonally open marine conditions were established, associated with marginal sea-ice zones. The onset of the current general circulation system of the Ross Sea was tentatively set at 7.8 ka BP, as a consequence of the polynya and HSSW formation.

We demonstrate that the coastal bay near Cape Hallett is a promising location to obtain detailed paleoenvironmental records for paleoclimate reconstructions. In this regard, the database should be further improved: the sediment core did not recover the base of the laminated layer, equivalent to the onset of the early Holocene warming. Based on the SBP seismic profile, the maximum thickness of the laminated sequence is expected to be 4 to 5 m thick. In addition, this study was based on the analyses and interpretation of a single sediment core. It is necessary to sample further sites in the bay in order to understand to what extent of confidence our paleoenvironmental interpretations can be extrapolated on a spatial scale. Finally, our findings have to be more closely integrated with regional information from the Ross Sea.

Acknowledgments Research carried out within the framework of the Project 4.5 (P.I. Prof. A. Brambati) of the Italian Programma Nazionale di Ricerche in Antartide, and financially supported by ENEA. We thank the crew and scientific party onboard R/V Italica

for their help with fieldwork during the ANTA02 cruise. The two referees Amy Leventer and Ross D. Powell are fully acknowledged for the critical review of the manuscript. The authors wish to dedicate this paper to the seaman David Basciano, who has left us too early. This is contribution No. 1434 of the ISMAR-CNR, Sezione Geologia Marina di Bologna, Italy.

References Anderson, J.B., 1999. Antarctic Marine Geology. Cambridge University Press, Cambridge (UK), 289 pp. Andrews, J.T., Jull, A.J.T., Leventer, A., 1997. Replication of accelerator mass spectrometry carbon-14 dates on the acidinsoluble fraction of Ross Sea surface sediments. Antarctic Journal of the United States 32, 37 – 38. Andrews, J.T., Domack, E.W., Cunningham, W.L., Leventer, A., Licht, K.J., Jull, A.J.T., DeMaster, D.J, Janning, A.E., 1999. Problems and possible solutions concerning radiocarbon dating of surface marine sediments, Ross Sea, Antarctica. Quaternary Research 52, 206 – 216. Baroni, C., Orombelli, G., 1994. Abandoned penguin rockeries as Holocene paleoclimatic indicators in Antarctica. Geology 22, 23 – 26. Berkman, P.A., 1997. Retreat of the West Antarctic Ice Sheet and subsequent persistence of marine macrofaunal assemblages along the Victoria Land coast during the Holocene. Antarctica and Global Change: Interactions and Impacts, Hobart, Tasmania Australia, 13–18 July 1997, Abstract, vol. 0805. Berner, R.A., 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York, 240 pp. Bonaccorsi, R., Brambati, A., Busetti, M., Fanzutti, G.P., 2000. Relationship among X-Rays lithofacies, magnetic susceptibility, P-waves velocity, and bulk density in core ANTA95-89 (Ross Sea, Antarctica). Terra Antartica Reports 4, 241 – 258. Boulton, G.S., 1990. Sedimentary and sea level changes durino glacial cyclesand their control on glacimarine architecture. In: Dowdeswell, J.A., Scourse, J.D. (Eds.), Glacimarine Environments: Processes and Sediments. Geological Soc. Spec. Publ., vol. 53, pp. 15 – 52. Brambati, A., Fanzutti, G.P., Finocchiaro, F., Melis, R., Frignani, M., Ravaioli, M., Setti, M., 1997. Paleoenvironmental record in Core ANTA91-30 (Drygalski Basin, Ross Sea, Antarctica). In: Cooper, A.K., Barker, P.F., Brancolini, G. (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin, Part 2. Antarctic Res. Series, vol. 71. Am. Geophys. Union, Washington, DC, pp. 137 – 151. Brambati, A., Corradi, N., Finocchiaro, F., Giglio, F., 2002. The position of the last glacial maximum grounding line in the Joides Basin. Royal Society of New Zealand Bulletin 35, 365 – 372. Brancolini, G., Harris, P.T., Shipboard Party, 2000. Post-cruise report: joint Italian/Australian marine geoscience expedition aboard the R.V. Tangaroa to the George V Land Region of East

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206 Antarctica during February–March 2000. AGSO Record No.2000/19, 181 pp. Bussi, M., Colizza, E., Corradi, N., Finocchiaro, F., Fontolan, G., Ivaldi, R., Landucci, C., Nicotra, G., Pitta’, A., Salvi, G., 2003. Late Quaternary palaeoclimatic evolution of marine sediments in the Southern Ocean project: preliminary results of the 20012002 PNRA cruise in the Ross Sea. Terra Antartica Reports 9, 89 – 94. Cias, P., Petit, J.R., Jouzel, J., Lorius, C., Barkov, N.I., Lipenkov, V., Nicolaiev, V., 1992. Evidence from an early Holocene climatic optimum in the Antarctic deep ice-core record. Climate Dynamics 6, 169 – 177. Cofaigh, C.O., Dowdswell, J.A., 2001. Laminated sediments in glacimarine environments: diagnostic criteria for their interpretation. Quaternary Science Reviews 20, 1411 – 1436. Colizza, E., Finocchiaro, F., Ivaldi, R., Pitta`, A., Tolotti, R., Brambati, A., 2003. Northern Victoria Land (Western Ross Sea—Antarctica): inner shelf fine sedimentation. Geo. Res. Abst. vol. 5. EGS p. 11853. Crawford, R.M., Hinz, F., Honeywill, C., 1998. Three species of the diatom genus Corethron Castracane: structure, distribution and taxonomy. Diatom Research 13 (1), 1 – 28. Cunningham, W.L., Leventer, A., 1998. Distribution of diatom assemblages in surface sediments of the Ross Sea, Antarctica: relationship to modern oceanographic conditions. Antarctic Science 10 (2), 134 – 146. Cunningham, W.L., Leventer, A., Andrews, J.T., Jennings, A.E., Licht, K.J., 1999. Late Pleistocene–Holocene marine conditions in the Ross Sea, Antarctica: evidence from the diatom record. The Holocene 9, 129 – 139. DeMaster, D.A., 1981. The supply and accumulation of silica in the marine environment. Geochimica et Cosmochimica Acta 45, 1715 – 1732. DeMaster, D.J., Raguenau, O., Nittrouer, C.A., 1996. Preservation efficiencies and accumulation rates for biogenic silica and organic C, N, and P in high-latitude sediments: the Ross Sea. Journal of Geophysical Research 101 (C8), 18501 – 18518. Denton, G.H., Bockheim, J.G., Wilson, S.C., Stuiver, M., 1989. Late Wisconsin and early Holocene glacial history, inner Ross embayment, Antarctica. Quaternary Research 3, 151 – 182. Domack, E.W., 1988. Biogenic facies in the Antarctic glacimarine environment: basis for a polar glacimarine summary. Palaeogeography, Palaeoclimatology, Palaeoecology 63, 357 – 372. Domack, E.W., Anderson, J.B., 1983. Marine geology of the George V continental margin: combined results of Deep Freeze 79 and the 1911–1914 Australian expedition. In: Olivier, R.L., James, P.R., Jago, J.B. (Eds.), Antarctic Earth Science. Australian Academy of Science. Cambridge University Press, Canberra, pp. 402 – 406. Domack, E.W., Harris, P., 1998. A new depositional model for ice shelves, based upon sediment cores from the Ross Sea and the Mac, Robertson shelf, Antarctica. Annals of Glaciology 27, 281 – 285. Domack, E.W., McClennen, C.E., 1996. Accumulation of glacial marine sediments in fjords of the Antarctic Peninsula and their

205

use as late Holocene paleoenvironmental indicators. Antarctic Research Series, A.G.U. 70, 135 – 154. Domack, E.W., Ishman, S.E., Stein, A.B., McClennen, C.E., Jull, T.A.J., 1995. Late Holocene advance of the Mqller Ice Shelf, Antarctic Peninsula: sedimentological, geochemical and paleontological evidence. Antarctic Science 7, 159 – 170. Domack, E.W., Jacobson, E.A., Shipp, S., Anderson, J.B., 1999. Late Pleistocene–Holocene retreat of the West Antarctic ice sheet system in the Ross Sea: part 2. Sedimentological and stratigraphic signature. GSA Bulletin 111, 1517 – 1536. Domack, E.W., Leventer, A., Dunbar, R.B., Taylor, F., Brachfeld, S., Sjunneskog, C., 2001. Chronology of the Palmer Deep site, Antarctic Peninsula: a Holocene palaeoenvironmental reference for the circum-Antarctic. The Holocene 11, 1 – 9. Dunbar, R.B., Anderson, J.B., Domack, E.W., Jacobs, S.S., 1985. Oceanographic influences on sedimentation along the Antarctic continental shelf. In: Jacobs, S.S. (Ed.), Oceanology of the Antarctic Continental Shelf. Antarctic Research Series, A.G.U., vol. 43, pp. 291 – 312. Finocchiaro, F., Melis, R., Tosato, M., 2000. Late Quaternary environmental events in two cores form southern Joides Basin (Ross Sea, Antarctica). Terra Antartica Reports 4, 125 – 130. Finocchiaro, F., Langone, L., Colizza, E., Busetti, M., Fontolan, G., Giglio, F., 2003. Preliminary results on a laminated sediment core collected from Cape Hallett bay (northern Victoria Land). Terra Antartica Reports 9, 105 – 108. Frignani, M., Giglio, F., Langone, L., Ravaioli, M., Mangini, A., 1998. Late-Pleistocene–Holocene sedimentary fluxes of organic carbon and biogenic silica in the northwestern Ross Sea, Antarctica. Annals of Glaciology 27, 697 – 703. Gingele, F.X., Kuhn, G., Maus, B., Melles, M., Schfne, T., 1997. Holocene ice retreat from the Lazarev Sea Shelf, East Antarctica. Continental Shelf Research 17, 137 – 163. Goodridge, C., 1999–2000. Mid Holocene warmth in the Antarctic Peninsula analog to global warming? Colgate University Journal of the Sciences 32, 67 – 84. Grigorov, I., Pearce, R.B., Kemp, A.E.S., 2002. Southern Ocean laminated diatom ooze: mat deposits and potential for palaeoflux studies, ODP leg 177, Site 1093. Deep Sea Research II 49, 3407 – 3991. Harris, P.T., 2000. Ripple cross-laminated sediments on the East Antarctic Shelf: evidence for episodic bottom water production during the Holocene? Marine Geology 170, 317 – 330. Harris, P.T., O’Brien, P.E., Sedwick, P., Truswell, E.M., 1996. Late Quaternary history of sedimentation on the McRoberson shelf, East Antarctica; problems with 14C-dating of marine sediments cores. Papers and Proceedings of the Royal Society of Tasmania 130, 47 – 53. Harris, P.T., Brancolini, G., Armand, L., Busetti, M., Beaman, R.J., Giorgetti, G., Presti, M., Trincardi, F., 2001. Continental shelf drift deposit indicates non-steady state Antarctic bottom water production in the Holocene. Marine Geology 179, 1 – 8. Hodell, D.A., Kanfoush, S.L., Shemesh, A., Crosta, X., Guilderson, T.P., 2001. Abrupt cooling of Antarctic surface waters and sea ice expansion in the South Atlantic sector of the Southern Ocean at 5000 cal yr B.P. Quaternary Research 56, 191 – 198.

206

F. Finocchiaro et al. / Global and Planetary Change 45 (2005) 193–206

Kellogg, T.B., Osterman, L.E., Stuiver, M., 1979. Late Quaternary sedimentology and benthic foraminiferal paleoecology of the Ross Sea, Antarctica. Journal of Foraminiferal Research 9 (4), 322 – 335. Langone, L., Frignani, M., Labbrozzi, L., Ravaioli, M., 1998. Present-day biosiliceous sedimentation in the NW Ross Sea (Antarctica). Journal of Marine Systems 17, 459 – 470. Leventer, A., Dunbar, R., DeMaster, D., 1993. Diatom evidence for Late Holocene climatic events in Granite Harbor, Antarctica. Paleoceanography 8, 373 – 386. Leventer, A., Domack, E.W., Ishman, S.E, Brachfeld, S., McClennen, C.E., Manley, P., 1996. Productivity cycles of 200–300 years in the Antarctic Peninsula region: understanding linkages among the sun, atmosphere, oceans, sea ice, and biota. GSA Bulletin 108, 1626 – 1644. Leventer, A., Domack, E.W., Barkoukis, A., McAndrews, B., Murray, J., 2002. Laminations from the Palmer Deep: a diatombased interpretation. Paleoceanography 17, 1 – 15. Licht, K.J., Andrews, J.T., 1997. Diamictons of the east-central Ross Sea continental shelf: implications for ice sheet extent during the last glacial maximum (LGM). Antarctica and Global Change: Interactions and Impacts, Hobart, Tasmania Australia, 13–18 July, Abstract vol. 0378. Licht, K.J., Jennings, A.E., Andrews, J.T., Williams, K.M., 1996. Chronology of late Wisconsin ice retreat from the western Ross Sea, Antarctica. Geology 24, 223 – 226. Licht, K.J., Dunbar, N.W., Andrews, J.T., Jennings, A.E., 1999. Distinguishing subglacial till and glacial-marine diamictons in the western Ross Sea, Antarctica: implications for a last glacial maximum grounding line. GSA Bulletin 111, 91 – 103. Lucchi, R.G., Rebesco, M., Camerlenghi, A., Busetti, M., Tomadin, L., Villa, G., Persico, D., Morigi, C., Bonci, M.C., Giorgetti, G., 2002. Mid-late Pleistocene glacimarine sedimentary processes of a high-latitude, deep-sea sediment drift (Antarctic Peninsula Pacific margin). Marine Geology 189, 343 – 370. Masson, V., Vimeux, F., Jouzel, J., Morgan, V., Delmotte, M., Cias, P., Hammer, C., Johnsen, S., Lipenkov, V.Y., Mosley-Thompson, E., Petit, J., Steig, E.J., Stievenard, M., Vaikmae, R., 2000. Holocene climate variability in Antarctica based on 11 ice-core isotopic records. Quaternary Research 54, 348 – 358. Melis, R., Colizza, E., Pizzolato, F., Rosso, A., 2002. Preliminary study of the calcareous taphocoenoses in Late Quaternary glacial marine sequences of the Ross Sea (Antarctica). Geobios 24, 207 – 218.

Nishimura, A., Nakasone, T., Hiramatsu, C., Tanahashi, M., 1998. Late Quaternary paleoenvironment of the Ross Sea continental shelf, Antarctica. Annals of Glaciology 27, 275 – 280. Orsini, G., Giglio, F., Langone, L., Ravaioli, M., 2003. Palaeoenvironmental inferences from Core ANTA95-1 (Granite Harbor, SW Ross Sea—Antarctica). Terra Antartica Reports 8, 133 – 138. Presti, M., De Santis, L., Busetti, M., Harris, P.T., 2003. Late Pleistocene and Holocene sedimentation on the George V Continental Shelf, East Antarctica. Deep Sea Research II 50 (8/9), 1441 – 1461. Pudsey, C.J., Camerlenghi, A., 1998. Glacial–interglacial deposition on a sediment drift on the Pacific margin of the Antarctic Peninsula. Antarctic Science 10 (3), 286 – 308. Pudsey, C.J., Barker, P.F., Larter, R.D., 1994. Ice sheet retreat from the Antarctic Peninsula shelf. Continental Shelf Research 14, 1647 – 1675. Reineck, H.E., Singh, I.B., 1973. Depositional Sedimentary Environments. Springer-Verlag, Berlin, 439 pp. Shipp, S.S., Anderson, J.B., 1994. Late Pleistocene deglaciation of Ross Sea (Antarctica) inferred from high-resolution seismic data. Abstracts with Programs-GSA 26, A-364. Siegert, M.J., 2001. Ice Sheets and Late Quaternary Environmental Change. John Wiley and Sons, Chichester (UK), 231 pp. Steig, E.J., Hart, C.P., White, J.W.C., Cunningham, W.L., Davis, M.D., Saltzman, E.S., 1998. Changes in climate, ocean and icesheet conditions in the Ross embayment, Antarctica, at 6 ka. Annals of Glaciology 27, 305 – 310. Stuiver, M., Denton, G.H., Hughes, T.J., Fastook, J.L., 1981. History of the marine ice sheet in West Antarctica during the LGM: a working hypothesis. In: Denton, G.H., Hughes, T.J. (Eds.), The Last Great Ice Sheets. John Wiley, New York, pp. 319 – 349. Taylor, F., McMinn, A., 2001. Evidence from diatoms for Holocene climatic fluctuation along the East Antarctic margin. The Holocene 11, 455 – 466. Taylor, F., Whitehead, J., Domack, E., 2001. Holocene paleoclimate change in the Antarctic Peninsula: evidence from the diatom, sedimentary and geochemical record. Marine Micropaleontology 41, 25 – 43. U.S.G.S., Map SS 58-60/2 Cape Hallett, 1:250.000 Antarctica Reconnaisance Series, 1968. van Beek, P., Reyss, J.-L., Paterne, M., Gersonde, R., van der Loeff, M., Kuhn, G., 2002. 226Ra in barite: absolute dating of Holocene Southern Ocean sediments and reconstruction of sea-surface reservoir ages. Geology 30, 731 – 734.