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During the last glacial maximum, a grounded ice sheet extended to (or close to) the edge of the continental shelf in the Ross Sea embayment (Stuiver et al. 1981; Anderson et al. 1992). Here we present several independent lines of geologic evidence suggesting that recession of the ice sheet from the western Ross Sea did not occur until 6,600-7,944 years ago as determined by carbon-14 (14C) dating.
A flow line of the ice sheet grounded on the Ross Sea floor extended westward around the northern tip of Ross Island and across McMurdo Sound ( figure 1). This ice plugged the mouth of Taylor Valley, damming Glacial Lake Washburn. Blue-green algae that once lived in the lake are now preserved within moraines and deltas that formed in Glacial Lake Washburn. From radiocarbon dates of such algae, we have determined that thick grounded ice blocked the mouth of Taylor Valley between 8,340 and 23,800 14C years ago. Kenyte erratics in Taylor Valley drift dated at 9,300 14C years ago confirm the late existence of the flow line crossing McMurdo Sound from Ross Island.
During the last glaciation, extensive closed-basin lakes also existed in Wright and Victoria Valleys (figure 1). We argue that these lakes, along with Glacial Lake Washburn, owe their existence to a grounded ice sheet in the Ross Sea, not only because it physically dammed some lakes but also because its presence is linked to increased meltwater production in the dry valleys. Glacial meltwater production in the dry valleys is sensitive to local radiation, which is highest during clear weather. Beneath a thin ice layer on the surface of the glaciers, solar radiation causes melting along intercrystalline boundaries to a depth of up to 1 meter (Hendy et al. in preparation). A network of drainage channels fed by the intercrystalline meltwater forms on and just below the glacier surface. This melt process accelerates with long stretches of clear weather because the intercrystalline meltwater does not freeze completely every evening (Hendy et al. in preparation). Today, open water in the adjacent Ross Sea is conducive to the formation of clouds, fog, and snow, which penetrate the dry valleys. Such conditions restrict meltwater production by reducing the length of clear, snowless weather, as well as by covering the glacier snouts with snow that shuts off the melt mechanism. For example, one heavy snowstorm in October 1977 terminated meltwater flow to Lake Vanda for an entire summer (Chinn 1981). The presence of grounded ice in the western Ross Sea would eliminate this local moisture source and thereby increase the length and frequency of clear, snowless weather in the dry valleys. In turn, this would boost radiation-induced meltwater production and, hence, raise lake levels. In this fashion, the high lake levels of late Wisconsin and early Holocene time imply increased aridity and radiative meltwater production tied to an extensive grounded ice sheet in the Ross Sea embayment. High lake levels were sustained 8,340-23,800 14C years ago in Taylor Valley; 7,944-25,697 14C years ago in Wright Valley; and 8,990-18,900 14C years ago in Victoria Valley. In Wright Valley, lake level dropped rapidly after 7,944 14C years ago, probably as a response to deglaciation in the western Ross Sea and the consequent penetration of moisture-bearing clouds into the dry valleys. Lakes in the dry valleys have fluctuated since 7,944 14C years ago, but they never have come close to attaining the high levels of late Wisconsin-early Holocene time.
Radiocarbon dates of marine shells, as well as of seal remains, afford close minimum ages for deglaciation of grounded ice in the western Ross Sea. The oldest individual shells and barnacles from McMurdo Sound date to 6,550-6,600 14C years ago (Stuiver et al. 1981; Kellogg, Kellogg, and Stuiver 1990; Gordon and Harkness 1992; Licht et al. 1996; all dates of marine material are corrected for an estimated 1,200-year marine reservoir effect). In addition, dates of shells from raised beaches and marine deposits along the Scott Coast are no older than 5,580 14C years ago ( figure 2; Stuiver et al. 1981, pp. 319-436; Hall and Denton in preparation). Our preliminary relative sea-level curve indicates that recession of grounded ice occurred along the Scott Coast south of Cape Ross shortly before 6,400 14C years ago (Hall and Denton in preparation; figure 3). The oldest date of an in situ shell in raised marine sediments at Terra Nova Bay (350 kilometers north of McMurdo Sound) is 6,305 14C years old (Baroni and Orombelli 1991) and affords a minimum age for deglaciation of grounded ice.
In conclusion, we list independent lines of geological evidence from marine and terrestrial data that both suggest deglaciation of the western Ross Sea at 6,600-7,944 14C years ago. One implication of such late deglaciation is that the grounded ice sheet in the Ross Sea embayment was relatively impervious to most (76-88 percent) of the 105-121 meters of sea-level rise that accompanied the last glacial/interglacial transition (Fairbanks 1989; Peltier 1994). By our chronology, only the final pulse of sea-level rise in early Holocene time could have triggered ice retreat from the western Ross Sea. In this case, ice recession to the Siple Coast grounding line may have been due largely to dynamic processes internal to the ice sheet because deglacial sea-level rise was essentially accomplished by mid-Holocene time.
This work was supported by National Science Foundation grant OPP 91-18678. T. Cruikshank and B. Overturf assisted in the field. D. Kelly drafted the figures.
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