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Reconstructing Neogene Climate and Glacial History of Southern McMurdo Sound, Antarctica
<p>Understanding the glacial changes that have occurred in southern McMurdo Sound throughout the Neogene makes an important contribution to reconstructing Antarctic ice volume changes during past periods of climatic warmth, and provides insight into future possible response of the ice sheet. Fossiliferous glacimarine deposits previously identified throughout McMurdo Sound have provided inferences on past changes in ice volume and the implications for global sea level. This study investigates new stratigraphic sections comprising fossiliferous glacimarine sediments from two locations on the flanks of Mount Discovery and one on Brown Peninsula at ~150m above present day sea-level. The aim of this thesis is to undertake a sedimentological, facies and glacimarine sequence stratigraphic analysis together with a quantitative assessment of the constituent micro and macrofossils in order to determine depositional processes, changes in environment and implications for glacial variability in the southern McMurdo Sound. Up to four distinct sedimentary cycles are evident in the Mt Discovery sections with each cycle consisting of: 1. A basal glacial surface of erosion (GSE) or its correlative conformity (CC) seaward of the grounding line, displaying an abrupt transition from a more distal facies to a more proximal facies. 2. A sharp-based massive diamictite displaying physical intermixing of subjacent lithologies, intra formational clasts, soft sediment deformation features, clast rotation features, and a lack of bioturbation, interpreted as subglacial, or in very close proximity to a marine grounding line. In some cases stratified diamictites overly correlative conformities displaying clast alignment, graded beds, and weak decimeter scale parallel bedding in the matrix, interpreted as grounding-line proximal sediment gravity flows or rain-out from ice melt. 3. In some cases, the diamictite passes gradationally-upwards or is sharply overlain by a conglomerate representing appearance of glacimarine fluvio-deltaic deposition or debris flows as the glacier grounding line begins to retreat. 4. Conglomerates are overlain, often sharply, by hemipelagic laminated or massive mudstone and represent the most ice distal and marine part of the sequence at the interglacial minima. 5. A proglacial facies succession is sometimes preserved below the GSE or correlative conformity marking the top of the sequence and usually consists of a transition from mudstone facies into grounding zone proximal conglomerates during re-advance of the grounding line. Lithofacies analysis supported by sediment grainsize, the results of a foraminiferal census and macrofaunal identifications conducted on each facies imply deposition on the continental shelf in depths of up to 400 m, during oscillations in the proximity of a marine grounding line. Radiocarbon dating of constituent macrofauna (barnacle plates) reported only background ¹⁴C implying an age of deposition older than the Last Glacial Maximum. The present day elevation of the site and a model developed here using glacio-iostatic loading and unloading implies that the Mt Discovery sites have been above sea-level for approximately the last ~2.7Ma. Given their inferred association with Scallop Hill Formation and faunal similarity to sequences in the nearby ANDRILL drill cores, a Pliocene age is considered most likely. This finding is consistent with previous investigations that indicate a dynamic, sub-polar marine-terminating ice sheet margin in Southern McMurdo Sound during the Pliocene, with periods of open ice shelf free conditions potentially associated with more frequent regional collapse and retreat events of a marine-based ice sheet in the Ross Sea embayment.</p>
<p>Understanding the glacial changes that have occurred in southern McMurdo Sound throughout the Neogene makes an important contribution to reconstructing Antarctic ice volume changes during past periods of climatic warmth, and provides insight into future possible response of the ice sheet. Fossiliferous glacimarine deposits previously identified throughout McMurdo Sound have provided inferences on past changes in ice volume and the implications for global sea level. This study investigates new stratigraphic sections comprising fossiliferous glacimarine sediments from two locations on the flanks of Mount Discovery and one on Brown Peninsula at ~150m above present day sea-level. The aim of this thesis is to undertake a sedimentological, facies and glacimarine sequence stratigraphic analysis together with a quantitative assessment of the constituent micro and macrofossils in order to determine depositional processes, changes in environment and implications for glacial variability in the southern McMurdo Sound. Up to four distinct sedimentary cycles are evident in the Mt Discovery sections with each cycle consisting of: 1. A basal glacial surface of erosion (GSE) or its correlative conformity (CC) seaward of the grounding line, displaying an abrupt transition from a more distal facies to a more proximal facies. 2. A sharp-based massive diamictite displaying physical intermixing of subjacent lithologies, intra formational clasts, soft sediment deformation features, clast rotation features, and a lack of bioturbation, interpreted as subglacial, or in very close proximity to a marine grounding line. In some cases stratified diamictites overly correlative conformities displaying clast alignment, graded beds, and weak decimeter scale parallel bedding in the matrix, interpreted as grounding-line proximal sediment gravity flows or rain-out from ice melt. 3. In some cases, the diamictite passes gradationally-upwards or is sharply overlain by a conglomerate representing appearance of glacimarine fluvio-deltaic deposition or debris flows as the glacier grounding line begins to retreat. 4. Conglomerates are overlain, often sharply, by hemipelagic laminated or massive mudstone and represent the most ice distal and marine part of the sequence at the interglacial minima. 5. A proglacial facies succession is sometimes preserved below the GSE or correlative conformity marking the top of the sequence and usually consists of a transition from mudstone facies into grounding zone proximal conglomerates during re-advance of the grounding line. Lithofacies analysis supported by sediment grainsize, the results of a foraminiferal census and macrofaunal identifications conducted on each facies imply deposition on the continental shelf in depths of up to 400 m, during oscillations in the proximity of a marine grounding line. Radiocarbon dating of constituent macrofauna (barnacle plates) reported only background ¹⁴C implying an age of deposition older than the Last Glacial Maximum. The present day elevation of the site and a model developed here using glacio-iostatic loading and unloading implies that the Mt Discovery sites have been above sea-level for approximately the last ~2.7Ma. Given their inferred association with Scallop Hill Formation and faunal similarity to sequences in the nearby ANDRILL drill cores, a Pliocene age is considered most likely. This finding is consistent with previous investigations that indicate a dynamic, sub-polar marine-terminating ice sheet margin in Southern McMurdo Sound during the Pliocene, with periods of open ice shelf free conditions potentially associated with more frequent regional collapse and retreat events of a marine-based ice sheet in the Ross Sea embayment.</p>
<p>Large volumes of aeolian sand and dust are deflated from unconsolidated till deposits, and supraglacial debris surrounding McMurdo Sound, Antarctica. This material is transported offshore with windblown snow onto extensive winter-formed sea ice in the southwest Ross Sea, and is subsequently released into the water-column during summer sea ice breakup. Aeolian sediment samples were collected from a ~600 km² area of sea ice in western McMurdo Sound to determine the magnitude of deposition and identify sediment sources. A new 2-dimensional numerical aeolian sediment transport model (NaMASTE) tuned specifically for the McMurdo Sound area, was used to explore the ability of the local wind system to move sediment from source areas to sea ice and to determine the pattern and extent of aeolian sediment dispersal to the southwest Ross Sea. Debris deposits on the McMurdo Ice Shelf debris bands are the most dominant sediment source for the area. Unconsolidated deposits between Cape Bernacchi and Spike Cape, and the Taylor Valley mouth are significant secondary deposits. Mass accumulation rates varied between 0.15 g m⁻² y⁻¹ and 54.6 g m⁻² y⁻¹, equating to a background aeolian sediment accumulation rate, excluding extremely high values, of 1.14 ± 0.59 g m⁻² y⁻¹ for the McMurdo Sound coastal sea ice zone. This is 3–5 orders of magnitude more than global background dust fallout for the Ross Sea. Modal grain size is very-fine sand to coarse silt. Notably, much of this material is distributed in localised, high sand content plumes that are oriented downwind from source, with finer deposits found outside these zones. An average seafloor linear sedimentation rate of 0.2 cm ky⁻¹ is calculated for McMurdo Sound, which is minor compared to biogenic sedimentation for the region. This equates to ~0.7 Gg y⁻¹ aeolian sediment entering McMurdo Sound during sea ice melt. Application of NaMASTE successfully simulated the general aeolian sediment distribution pattern. Testing of model variables suggests that aeolian material is mainly transported during strong (>20 m s⁻¹) wind events. Modelling also suggests aeolian material from McMurdo Sound can be transported north to the Drygalski Ice Tongue, ~250 km from source, but only in very trace quantities.</p>
<p>Large volumes of aeolian sand and dust are deflated from unconsolidated till deposits, and supraglacial debris surrounding McMurdo Sound, Antarctica. This material is transported offshore with windblown snow onto extensive winter-formed sea ice in the southwest Ross Sea, and is subsequently released into the water-column during summer sea ice breakup. Aeolian sediment samples were collected from a ~600 km² area of sea ice in western McMurdo Sound to determine the magnitude of deposition and identify sediment sources. A new 2-dimensional numerical aeolian sediment transport model (NaMASTE) tuned specifically for the McMurdo Sound area, was used to explore the ability of the local wind system to move sediment from source areas to sea ice and to determine the pattern and extent of aeolian sediment dispersal to the southwest Ross Sea. Debris deposits on the McMurdo Ice Shelf debris bands are the most dominant sediment source for the area. Unconsolidated deposits between Cape Bernacchi and Spike Cape, and the Taylor Valley mouth are significant secondary deposits. Mass accumulation rates varied between 0.15 g m⁻² y⁻¹ and 54.6 g m⁻² y⁻¹, equating to a background aeolian sediment accumulation rate, excluding extremely high values, of 1.14 ± 0.59 g m⁻² y⁻¹ for the McMurdo Sound coastal sea ice zone. This is 3–5 orders of magnitude more than global background dust fallout for the Ross Sea. Modal grain size is very-fine sand to coarse silt. Notably, much of this material is distributed in localised, high sand content plumes that are oriented downwind from source, with finer deposits found outside these zones. An average seafloor linear sedimentation rate of 0.2 cm ky⁻¹ is calculated for McMurdo Sound, which is minor compared to biogenic sedimentation for the region. This equates to ~0.7 Gg y⁻¹ aeolian sediment entering McMurdo Sound during sea ice melt. Application of NaMASTE successfully simulated the general aeolian sediment distribution pattern. Testing of model variables suggests that aeolian material is mainly transported during strong (>20 m s⁻¹) wind events. Modelling also suggests aeolian material from McMurdo Sound can be transported north to the Drygalski Ice Tongue, ~250 km from source, but only in very trace quantities.</p>
<p>Each summer the waters in McMurdo Sound (Lat. 77.5ºS; Long. 165ºE), south-western (SW) Ross Sea encounter vast phytoplankton blooms. This phenomenon is stimulated by the addition of bio-available iron (Fe) to an environment where phytoplankton growth is otherwise Fe-limited. One possible source of such Fe is aeolian sand and dust (ASD) which accumulates on sea ice and is released into the ocean during the summer melt season. The amount of bio-available Fe (i.e. the amount of Fe immedately accessible to phytoplankton) potentially supplied to the ocean by ASD depends on a number of factors including; the ASD flux into the ocean, its particle size distribution and Fe content. However, none of these parameters are well constrained in the SW Ross Sea region and, as a result, the significance of this Fe source in the biogeochemical cycle of phytoplankton growth remains to be quantified. This study focuses on an area (7400 km²) of Southern McMurdo Sound, one of the few areas where direct sampling of ASD that has accumulated on sea ice is possible. To evaluate the flux and solubility of Fe contained in ASD into McMurdo Sound, the mass accumulation rate and particle size of 70 surface snow samples and 3 shallow (3 m) firn cores from the nearby McMurdo Ice Shelf covering the period 2000 - 2008 have been analysed. Selected samples were also measured for total and soluble Fe, Sr and Nd isotopic ratios and mineralogy as a guide to Fe-fertilisation potential and provenance, respectively. Mass and particle size data show an exponential decrease in mass accumulation rate (from 26.00 g m⁻² yr⁻¹ to 0.70 g m⁻² yr⁻¹) and a decrease in modal particle size (from 130 to 69 μm) over a distance of 120 km from Southern McMurdo Sound northwards to Granite Harbour. Both these trends are consistent with ASD being dispersed northwards across the sea ice by southerly storms from an area of the McMurdo Ice Shelf, where submarine freezing and surface ablation have resulted in a surface covered with debris from the sea floor, known as the 'dirty ice' or 'debris bands' (Lat. 77.929ºS; Long. 165.505ºE) in Southern McMurdo Sound. This assertion is further supported by the Sr and Nd isotopic signature of ASD matching local source rocks and the presence of vesicular glass of Southern McMurdo Sound in all samples which also points to the debris bands as the origin of ASD in McMurdo Sound. Bio-available Fe is extremely difficult to quantify hence Fe solubility was used as an approximation in this thesis. Analysis of both total (i.e. particulate and soluble) and the percentage of soluble Fe in the 0.4 - 10 μm dust size fraction (i.e. the fraction most likely to become bio-available) by solution ICP-MS shows a narrow range of values; 3.84 ± 1.99 wt % and 9.42 ± 0.70 % respectively. Combining these values with mass accumulation rate estimates for the particles 0.4 - 10 μm in size, gives an annual soluble Fe flux for the region 500 km² north of the debris bands in McMurdo Sound of 0.55 mg m⁻² yr⁻¹ (9.89 μmol m⁻² yr⁻¹), with spatial variability largely determined by differences in mass accumulation rate. These fluxes are at least an order of magnitude greater than predicted in global dust deposition models for the Southern Ocean and measured in snow samples from East Antarctica. Furthermore, these values exceed the Fe threshold, estimated as 0.2 nM (Boyd and Abraham, 2001), required for phytoplankton growth following the simple dust-biota model of Boyd et al. (2010) and assuming the release of captured ASD in snow is instantaneous. Whilst not constrained in the present study, ASD sourced from the debris bands may be sufficiently widely dispersed, particularly during storm years, to contribute to Fe-fertilisation up to 1200 km from Southern McMurdo Sound. Short, ~10 year long, firn core records of mass accumulation and methylsuphonate concentration, a proxy for phytoplankton productivity, shows a close correspondence between the two during particularly stormy years. Whilst not demonstrating a cause-and-effect relationship, this observation suggests coastal ice cores may contain an important record of the interplay between climate, dust supply, Fe-fertilisation of near shore waters and phytoplankton productivity on decadal and longer timescales.</p>
<p>Each summer the waters in McMurdo Sound (Lat. 77.5ºS; Long. 165ºE), south-western (SW) Ross Sea encounter vast phytoplankton blooms. This phenomenon is stimulated by the addition of bio-available iron (Fe) to an environment where phytoplankton growth is otherwise Fe-limited. One possible source of such Fe is aeolian sand and dust (ASD) which accumulates on sea ice and is released into the ocean during the summer melt season. The amount of bio-available Fe (i.e. the amount of Fe immedately accessible to phytoplankton) potentially supplied to the ocean by ASD depends on a number of factors including; the ASD flux into the ocean, its particle size distribution and Fe content. However, none of these parameters are well constrained in the SW Ross Sea region and, as a result, the significance of this Fe source in the biogeochemical cycle of phytoplankton growth remains to be quantified. This study focuses on an area (7400 km²) of Southern McMurdo Sound, one of the few areas where direct sampling of ASD that has accumulated on sea ice is possible. To evaluate the flux and solubility of Fe contained in ASD into McMurdo Sound, the mass accumulation rate and particle size of 70 surface snow samples and 3 shallow (3 m) firn cores from the nearby McMurdo Ice Shelf covering the period 2000 - 2008 have been analysed. Selected samples were also measured for total and soluble Fe, Sr and Nd isotopic ratios and mineralogy as a guide to Fe-fertilisation potential and provenance, respectively. Mass and particle size data show an exponential decrease in mass accumulation rate (from 26.00 g m⁻² yr⁻¹ to 0.70 g m⁻² yr⁻¹) and a decrease in modal particle size (from 130 to 69 μm) over a distance of 120 km from Southern McMurdo Sound northwards to Granite Harbour. Both these trends are consistent with ASD being dispersed northwards across the sea ice by southerly storms from an area of the McMurdo Ice Shelf, where submarine freezing and surface ablation have resulted in a surface covered with debris from the sea floor, known as the 'dirty ice' or 'debris bands' (Lat. 77.929ºS; Long. 165.505ºE) in Southern McMurdo Sound. This assertion is further supported by the Sr and Nd isotopic signature of ASD matching local source rocks and the presence of vesicular glass of Southern McMurdo Sound in all samples which also points to the debris bands as the origin of ASD in McMurdo Sound. Bio-available Fe is extremely difficult to quantify hence Fe solubility was used as an approximation in this thesis. Analysis of both total (i.e. particulate and soluble) and the percentage of soluble Fe in the 0.4 - 10 μm dust size fraction (i.e. the fraction most likely to become bio-available) by solution ICP-MS shows a narrow range of values; 3.84 ± 1.99 wt % and 9.42 ± 0.70 % respectively. Combining these values with mass accumulation rate estimates for the particles 0.4 - 10 μm in size, gives an annual soluble Fe flux for the region 500 km² north of the debris bands in McMurdo Sound of 0.55 mg m⁻² yr⁻¹ (9.89 μmol m⁻² yr⁻¹), with spatial variability largely determined by differences in mass accumulation rate. These fluxes are at least an order of magnitude greater than predicted in global dust deposition models for the Southern Ocean and measured in snow samples from East Antarctica. Furthermore, these values exceed the Fe threshold, estimated as 0.2 nM (Boyd and Abraham, 2001), required for phytoplankton growth following the simple dust-biota model of Boyd et al. (2010) and assuming the release of captured ASD in snow is instantaneous. Whilst not constrained in the present study, ASD sourced from the debris bands may be sufficiently widely dispersed, particularly during storm years, to contribute to Fe-fertilisation up to 1200 km from Southern McMurdo Sound. Short, ~10 year long, firn core records of mass accumulation and methylsuphonate concentration, a proxy for phytoplankton productivity, shows a close correspondence between the two during particularly stormy years. Whilst not demonstrating a cause-and-effect relationship, this observation suggests coastal ice cores may contain an important record of the interplay between climate, dust supply, Fe-fertilisation of near shore waters and phytoplankton productivity on decadal and longer timescales.</p>
<p>Modern beaches in McMurdo Sound can be divided into 3 process regimes. Beaches on Ross Island (eastern McMurdo Sound) are characterised by marine processes with little ice modification. On ice-bound western McMurdo Sound, coastal orientation is of paramount importance. Ice thrust features are prominent on south facing beaches, which are open to the predominant wind direction and receive relatively small waves from the fetch restricted south. A greater degree of marine dominance is exhibited by beaches on north facing coasts where sea ice is blown offshore and the beaches are open to the larger storm waves from the eastern Ross Sea. The single most useful indicator of the relative importance of marine and ice processes on the beaches is the roundness of the beach material. Unlike the modern beaches, raised beach ridges at all sites comprise poorly sorted cobbles in a mixed sand and gravel matrix. These are inferred to be storm ridges. In contrast with the raised beaches, the modern beaches on the western side of the Sound have evidence of ice processes on them. This suggests that the modem beach has not experienced the same magnitude storms that produced the raised beaches. The size and frequency of the ridges is a product of the local wave climate. The number of raised beaches at any site is a useful indicator of the paleo-wave climate. More ridges occur in sheltered south facing locations, because they are more protected from open marine conditions, than on beaches in ice-free or north facing locations. When determining the marine limit of a site the most useful features are, low energy marine bedding features (such as flaser bedding) and boulder pavements. Based on inferred process information at the time of deposition, revised estimates of marine limits in McMurdo Sound and a new marine limit at Cape Barne are presented. Because the nature of the raised beaches has not been fully considered by previous authors sea level curves are inaccurate. The reconstruction of the retreat of the Ross Ice Shelf from marine limits in McMurdo Sound shows a three stage stepwise southward retreat of the Ross Ice Shelf. A breakout from somewhere north of Cape Roberts and south of Cape Ross back to Marble Point (on the western side of the Sound) while remaining north of Cape Bird (on the eastern side of the Sound), occurred sometime around 8,000 years ago. Another breakout cleared ice from Cape Bird to somewhere south of Cape Barne and south of Cape Bernacchi around 5,000 years ago. This differs with other authors work (Hall and Denton, 1999, Kellogg et al., 1996, Stuiver et al., 1981) by suggesting a considerably older date for the Ross Ice Sheet retreating from McMurdo Sound. The data presented here suggests that much of McMurdo Sound was ice free about 1,500 years before earlier estimates at about 6,500 years. The effect of the change in deglaciation timing is to reduce isostatic rebound rates. This suggests that there was less ice in McMurdo Sound during the Last Glacial Maximum.</p>
<p>Modern beaches in McMurdo Sound can be divided into 3 process regimes. Beaches on Ross Island (eastern McMurdo Sound) are characterised by marine processes with little ice modification. On ice-bound western McMurdo Sound, coastal orientation is of paramount importance. Ice thrust features are prominent on south facing beaches, which are open to the predominant wind direction and receive relatively small waves from the fetch restricted south. A greater degree of marine dominance is exhibited by beaches on north facing coasts where sea ice is blown offshore and the beaches are open to the larger storm waves from the eastern Ross Sea. The single most useful indicator of the relative importance of marine and ice processes on the beaches is the roundness of the beach material. Unlike the modern beaches, raised beach ridges at all sites comprise poorly sorted cobbles in a mixed sand and gravel matrix. These are inferred to be storm ridges. In contrast with the raised beaches, the modern beaches on the western side of the Sound have evidence of ice processes on them. This suggests that the modem beach has not experienced the same magnitude storms that produced the raised beaches. The size and frequency of the ridges is a product of the local wave climate. The number of raised beaches at any site is a useful indicator of the paleo-wave climate. More ridges occur in sheltered south facing locations, because they are more protected from open marine conditions, than on beaches in ice-free or north facing locations. When determining the marine limit of a site the most useful features are, low energy marine bedding features (such as flaser bedding) and boulder pavements. Based on inferred process information at the time of deposition, revised estimates of marine limits in McMurdo Sound and a new marine limit at Cape Barne are presented. Because the nature of the raised beaches has not been fully considered by previous authors sea level curves are inaccurate. The reconstruction of the retreat of the Ross Ice Shelf from marine limits in McMurdo Sound shows a three stage stepwise southward retreat of the Ross Ice Shelf. A breakout from somewhere north of Cape Roberts and south of Cape Ross back to Marble Point (on the western side of the Sound) while remaining north of Cape Bird (on the eastern side of the Sound), occurred sometime around 8,000 years ago. Another breakout cleared ice from Cape Bird to somewhere south of Cape Barne and south of Cape Bernacchi around 5,000 years ago. This differs with other authors work (Hall and Denton, 1999, Kellogg et al., 1996, Stuiver et al., 1981) by suggesting a considerably older date for the Ross Ice Sheet retreating from McMurdo Sound. The data presented here suggests that much of McMurdo Sound was ice free about 1,500 years before earlier estimates at about 6,500 years. The effect of the change in deglaciation timing is to reduce isostatic rebound rates. This suggests that there was less ice in McMurdo Sound during the Last Glacial Maximum.</p>
<p>This thesis presents the results of a study of benthic foraminifera from McMurdo Sound, Antarctica. The sound is 50 km across and more than 900 m deep, and is ice-covered for at least 9 months of the year. However, salinity and temperature of the bottom waters are constant (35 per mil and -1.8 degrees C). Sea floor sediment is mainly fine sand and mud with a little ice-rafted gravel. The aim of the study was to document the distribution of living and dead foraminifera and to determine the factor(s) controlling it. The twenty-six sites in water from 76 to 856m deep were sampled by gravity corer and grab, and nearly 40,000 specimens (2334 living and 36,875 dead) were identified. Three present day assemblages can be recognised: 1. Shallow open water assemblage (SWA): Trochammina glabra, Cribrostomoides jeffreysii, Trifarina earlandi, Ehrenbergina glabra, Fursenkoina earlandi and Globocassidulina crassa. 2. Deep open water assemblage (DWA): Reophax pilulifer, Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica and Miliammina arenacea. 3. Harbour/enclosed basin assemblage (HA): Reophax subdentaliniformis, Portotrochammina antarctica, Textularia antarctica, Fursenkoina earlandi and Globocassidulina crassa. The composition of the assemblages is controlled largely by calcium carbonate compensation depth (CCD). Calcareous species are abundant and varied (84 calcareous species) in the SWA above 620m, but are virtually absent from the DWA, which is found in deeper water. The dominance of agglutinated foraminifera in the HA indicates an even shallower CCD (about 270m) in restricted coastal settings. Death assemblages have a similar species diversity to corresponding life assemblages and are reasonably representative of them, except for the 200m zone above the offshore CCD, where death assemblages are depleted in calcareous taxa. The diversity of the agglutinated component of each assemblage remains nearly constant in all habitats and at all water depths, even though shallow water samples include a range of calcareous species. Thus competition from calcareous species appears not to be a stress factor for agglutinated species, which are considered to have reached the limit of their evolutionary potential in these waters.</p>
