It turned out that the age of Ellesmere Island ice shelves is estimated by driftwood located on the shore. an age of ~3,000 years is estimated for the Ward Hunt ice shelf (in which a crack recently developed) based on driftwood located onshore. I haven’t seen corresponding information on the Ayles ice shelf (which actually has broken up.)
In attempting to evaluate how much weight to put on absence of upbeach driftwood as an ice shelf dating method, I ran across a very interesting article, Dyke et al 1997, online here entitled Changes in Driftwood Delivery to the Canadian Arctic Archipelago: The Hypothesis of Postglacial Oscillations of the Transpolar Drift, which sheds light on how much weight to place on presence/absence of driftwood upbeach of modern ice shelves.
Before discussing the interesting questions of the Transpolar Drift (an Arctic current), I’ll first present Dyke et al Figure 4 which bears fairly directly on assessing driftwood presence/absence as an ice shelf dating method. Their Figure 4 shows a census of radiocarbon dated driftwood from the north shore of Ellesmere Island in 250-year intervals. In the last 12 250-year periods going back to 3000 BP, 4 periods had 0 pieces of driftwood and 5 periods had only 1 piece of driftwood.
So, as a start, there would be a considerable randomness to the presence/absence of piece of driftwood in any given 250-year period and it would be extremely difficult to place a lot of statistical significance on the absence of a piece of MWP driftwood purely on statistical grounds. (There are some other issues relating to E-W movement of the Transpolar Drift and to driftwood in Svalbard that I’ll get to on another occasion.)
Dyke et al 1997 FIG. 4. Frequency distribution of radiocarbon dates on driftwood from the Arctic Ocean coast of Ellesmere Island. Samples that date “modern” are plotted to the left of 0 years B.P.Now there’s one sample in this diagram that really catches my eye. Most of the samples dated younger than 3000 years are not behind present ice shelves, but one of them is. I wonder where that sample comes from. This one sample could have quite an impact on how one interprets the driftwood information, given the relative scarcity of samples.
Obviously the existence of a “young” piece of driftwood proves that it was delivered upbeach somehow. Given the scarcity of samples, the absence of driftwood “suggests” or “hints” or “indicates” that the ice shelf was present at intervening times (and thus preventing delivery of driftwood upbeach), but the extremely small population of relevant samples leaves open a very significant possibility that the ice shelf could have been temporarily absent, but that no driftwood happened to be delivered upbeach during the open window. Now no one has suggested that temperatures in this area in the MWP were as high as during the Holocene Optimum and, as noted above, even with the crack in the Ward Hunt ice shelf, it would not be possible at present to deliver driftwood upbeach of the Ward Hunt ice shelf anyway.
The process of delivering driftwood to the north coast of Ellesmere Island also raises many interesting issues. Dyke et al observed that virtually all driftwood to the north coast of Ellesmere Island comes from Siberia. This is proven by the ratio of larch to spruce – larch grows in Asia, spruce in North America. Ellesmere Island has a very high larch ratio. They say that the driftwood is incorporated into sea ice in the polynas offshore Siberia (which turn out to be the main “ice factories” in the Arctic – another interesting byway) and that the sea ice transports driftwood across the Arctic in the Trans-Polar Drift.
Dyke et al hypothesize that the terminus of the Trans-Polar Drift has varied considerably through the Holocene. In one extreme, the Trans-Polar Drift exits the Arctic in the Fram Strait east of Greenland and delivers little or no driftwood to the Canadian Archipelago; in another extreme, the Trans-Polar Drift turns west and delivers driftwood to the archipelago.
Dyke et al 1997 FIG. 8. A model for interpreting the driftwood chronologies of Arctic Canada. Configurations A and B produce negatively correlated wood abundances in the two major regions. Configuration C produces positively correlated abundances.
Here is the abstract from Dyke et al 1997:
Driftwood appears to be absent in the Beaufort Gyre but abundant in parts of the Transpolar Drift (TPD), which crosses the Arctic Ocean from the Chukchi Sea to the vicinity of northeastern Greenland. Nearly 300 radiocarbon dates on Holocene driftwood from the Canadian Arctic Archipelago reveal two regions with contrasting histories of driftwood incursion: the region accessible to wood brought into Baffin Bay by the West Greenland Current and the rest of the archipelago, which receives wood from the Arctic Ocean. We hypothesize that when the TPD was deflected westward along northern Greenland, wood was delivered widely to the Canadian Arctic Archipelago; when the TPD exited entirely through Fram Strait via the East Greenland Current, little or no wood was delivered to most of the archipelago, but some continued into Baffin Bay via the West Greenland Current. A split TPD delivered wood to both regions. The regional driftwood incursion histories exhibit multiple maxima and minima that can be explained by this hypothesis. The Larix to Picea ratio of wood arriving in the Canadian Arctic Archipelago has also changed through time. This may indicate varying contributions from Russian versus North American sources, which in turn may indicate variable mixing of wood en route. The inferred discharge paths of the TPD were apparently stable for intervals ranging from several millennia to centuries or perhaps only decades. The last major switch broadly correlates with the onset of Neoglaciation.
Elsewhere they discuss the delivery of driftwood to Ellesmere Isalnd observing the following:
The aggregate distribution of driftwood dates from the Arctic Ocean coast of Ellesmere Island resembles that for the Baffin Bay region (cf. Figs. 3 and 4). However, this similarity is largely due to the effort applied to dating wood behind the ice shelves. When only samples from areas without ice shelves are considered, important differences appear between the records of the two regions. For example, the decline of wood abundance during the late Holocene, a prominent feature of the Baffin Bay record, is not apparent here (cf. Figs. 3 and 4). This part of the archipelago is nearest to wood sources, and the two oldest dated wood samples from the entire archipelago are from here, both from 8.9 ka B.P. (Stewart and England, 1983; Bednarski, 1986; Lemmen, 1988). Driftwood arrived in only moderate abundance until 6.75 ka B.P., when it increased coincidentally with its increase in Jones Sound. Prominent modes of driftwood arrival date from 6 to 5.75 ka B.P. and from 4.75 to 4.5 ka B.P. The strong mode in Jones Sound between 5.25 and 5 ka B.P. correlates with a minimum in the northern Ellesmere record (cf. Figs. 3 and 4).
Driftwood arrived sparsely between 8.6 and 6 ka B.P. when it suddenly, but briefly, increased to one of two middle Holocene maxima, between 6 and 5.75 ka B.P. This modal abundance correlates exactly with the mode for the northern coast of Ellesmere Island (cf. Figs. 4 and 6). Thus this brief event of abundant wood arrival is widely recorded. Similarly, both records display driftwood minima between 5.25 and 5 ka B.P. that, in turn, correlate with the modal abundance in the Baffin Bay region (cf. Figs. 4 and 6 with 3). The mode between 4.75 and 4.25 ka B.P. in the central Arctic appears to be represented in both the northern Ellesmere and the Baffin Bay records. The strong central Arctic mode between 3.75 and 3.5 ka B.P. corresponds with the sharp decline in wood abundance in the Baffin Bay region.
There’s some interesting information on driftwood supply to Svalbard that I’ll try to summarize on another occasion – there’s a MWP peak, which may or may not represent a fluctuation in the Transpolar Drift terminus. While I was doing this, I also noticed some very interesting information on Arctic Ocean paleoclimate proxies (and especially about the relative warmth of the subsurface Arctic Ocean and the reverse thermocline) – again a topic for another day.
So what can we conclude about relative modern-medieval temperatures based on information from the Ayles and Ward Hunt ice shelves based on the evidence so far (and I’ve still got original references to run down on the driftwood samples)? There’s a crack in the Ward Hunt ice shelf – which in itself does not say anything about modern-medieval relationships. None of the “unprecedented” articles give any driftwood information on the Ayles Ice Shelf which has broken up.
There’s at least one “young” radiocarbon-dated piece of driftwood that has been located upbeach of a modern ice shelf. So there’s more to come on this topic.
ARTHUR S. DYKE, JOHN ENGLAND, ERK REIMNITZ and HàÆ”¬°LàÆà’ NE JETTàÆ”¬°, 1997. Changes in Driftwood Delivery to the Canadian Arctic Archipelago: The Hypothesis of Postglacial Oscillations of the Transpolar Drift, ARCTIC 50, 1–16 nn http://pubs.aina.ucalgary.ca/arctic/Arctic50-1-1.pdf