At °S, ice sheet deglaciation after the local Last Glacial Maximum (LGM) . This paper produces a new glacial varve chronology through Lago Buenos Aires outlet glacier is the numerical simulation of Hubbard et al. Annually laminated sediments (glacial varves) from Lake Silvaplauna, a High Alpine annually since but their contribution is negligible before this date. dating of the cores, partly through available meteorological and Key words: Sedimentation, varves, glacier lake, bed load, cli- kins and Sims ). To date.
The mode of ice melting, subglacial drainage and esker formation with 3 esker centra yellow and 3 successive annual varves pink. From field observations to chronological tools The varves are observed and recorded in open pits or in cores. An open pit is always better because it allow us to view the lateral variations.
In cores, very long and continual sequences can be obtained, however. De Geer introduced the simple method of rolling out a paper stripe over the section or core of varves and marking each individual varve on the stripe.
Then the individual varve thicknesses were measured and plotted on a diagram. The saw-tooth patterns of the varve diagrams were then used for inter-site correlations.
Even visually, it is easy to see the nearly identical variations in varve thickness. The varve diagram shows variations that allow the correlation with the main Swedish Time Scale, so that absolute ages are obtained. Varved clay cores take two years apart at approximately the same site.
The inter-core correlations are very clear. The entire section includes varves. Via the marker varves, the section can be correlated to the Swedish Time Scale and dated in absolute varve ages BP. By this, the varve chronology was fixed to the present and we were able to talk in terms of absolute years.
Fromm measured pollen and diatoms in the same varves, implying that we from that time on were able to know the absolute ages of the immigration of different tree species, and the changes between fresh-water and marine stages of the Baltic. All this was, of course, quite remarkable at a time period were we generally lacked other means of establishing absolute time.
The varve chronology flourished also in Finland Sauramo,and was also applied to eastern North America by Antevs e. A period of hesitation and change of focus With the introduction of the radiocarbon dating method Arnold and Libby, things changed, and there suddenly was an alternative method of obtaining absolute ages.
Also, quite bad errors in the varve ages were documented; especially in eastern US and Canada e. Ridge and Larsen, Internationally, the application of varve dating, rather switched from the ice recessional records in Sweden De Geer,Finland Sauramo, and North America Antevs, to chronologies of continual lake records.
Annually varved sediments were discovered in a large number of non-glacial lakebeds from other parts of the world. This opened for local absolute dating of lake deposits. Many excellent papers were published e.
A period of revision and extension In Sweden and Finland, we entered into a period of revision.
So, today, the Swedish Varve Chronology spans some 14, varves Fig. The missing varves must be searched for at a time younger than varves BP, and maybe between and BP Wohlfarth et al.
In Estonia and the St. The application of events, spatial distribution and rates Varve dating is very useful when it concerns the dating of the duration of a geological event. De Geer was able to show the mode of ice recession and date esker centra and moraine ridges as to single years Fig. Varve chronology also gives the background for rate calculations. The classical example is the rate of ice retreat and its changes over time De Geer, The rate of ice marginal recession over the Stockholm area was in the order of m per year, despite the fact that the ice flow to the front was in the order of m per year, implying a total annual melting of about m.
This is an enormous rate of ice melting Fig. The deglaciation of the Ontario region in Canada is characterized by a number of end moraines representing halts or minor re-advances. By applying a relative varve chronology, it was possible to date the duration of the building up of the Tillsonburg Moraine at 22 years Fig. A varve sequence right in front of the Tillsonburg-Sparta I end-moraine includes 98 varves; 57 recessional varves, 19 readvance varves and 22 ice-marginal varves.
This indicates a readvance in the order of 8 km and time of ice-marginal halt of only 22 years. In a few cases it has been possible also to pinpoint the season of an event. Because different events in Sweden could be tied to one and the same varve, it was possible to document the spatial distribution of those events. Turbidites were recorded at single varves and their spatial distribution recorded.
There is a relation between seismic magnitude and the spatial distribution of liquefaction. This is a very high seismic frequency or recurrence time. This record could only have been achieved thanks to the firm varve dating. This is a unique value, which provides a very accurate measurement of the rate of uplift right after the free-melting. Micro-varved postglacial lake sequences occur both in Sweden and Finland.
Cesium measurements were chasms 8—20 m deep in a lakebed highly dissected by hundreds made by gamma spectrometry at the St. Croix Watershed of smaller tributary gullies 1—5 m deep. While this subaerial Research Station, and AMS radiocarbon dating was done at the lakebed erosion created steep-walled, cohesive outcrops that Lawrence Livermore National Laboratory.
Radiocarbon dates reveal lacustrine stratigraphy recording over yr of were calibrated using CALIB version 5. We performed most grain- document that record was short-lived. Methods Moraines, glacier margins, lakebed bathymetry, shorelines, deltas, and sampling locations were mapped using kinematic We examined the Iceberg Lake lacustrine stratigraphic differential global positioning system DGPS equipment.
We record in broad outcrops, mostly without need for coring were unable to tie our survey to a local benchmark, so absolute equipment. At each outcrop, we first used shovels to expose position of the survey is poorly constrained. Maps and a digital elevation model were prepared from these Outcrops were measured and tagged at cm intervals counting data using the 3D spatial analyst extension in ArcGIS ESRI, down from the top of each section.
The bottom of each section We mapped and described geological features in the was in all cases dictated by practical limitations usually the field using aerial photos and published topographic maps, and inability to dig further in saturated, thixotropic muds rather we conducted irregular spot measurements of stream discharge than by the true lower boundary of a stratigraphic unit, and we and sediment concentrations in inlet streams and the partially- thus emphasize that the record presented in this manuscript is filled lake.
All fieldwork was conducted during summers incomplete: Sections were then logged and photographed, and all laminae were measured in the field using digital calipers. This Results approach presented several advantages over traditional sediment cores: Representative sediment of clear laminated couplets Fig. Before describing these samples were collected for later grain-size and bulk density couplets, presenting evidence that they are annual varves, and analyses, and the few organic materials found were sampled considering their climatic implications, we briefly describe a for radiocarbon dating.
Many are found as thick 1— cm; only the largest are 4 August 3 shown in Fig.
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Stratigraphic sections from twelve sites at Iceberg Lake. Marked depths are distances, in cm, below the top of the section. Site I is above the most recent stable lake level and was collected with a Livingstone-type Corer.
Using a combination of varve-counting, cross-dating, and radiogenic dating, we tied the sections shaded in red to a master chronology. Dating on other sections is uncertain.
Note that Sites H and L do not exist—these two breaks in the lettering system divide sites visited in, and At all sites, we also found occasional lenticular Laminated couplets and evidence for annual deposition pockets of coarse sand, gravel, and angular stones that were easily recognized as ice-rafted debris from the calving margin of We now turn our attention to the laminated couplets.
In both the glacier damming Iceberg Lake. Grain sizes of these strata mean grained summer deposit. Summer layers are more suggestion that they are reworked varves.
We used the more abrupt transition from varve sequences further downsection. The thickest and most winter to summer as a marker when measuring couplets. In contrast, some anomalous deposits occur basin-wide, serving as valuable marker horizons for strati- graphic correlation.
Episodic shoreline-lowering events, for example, produce transient increases in the basinwide deposition of fine-grained sediment, especially at delta- proximal sites Loso et al. Another important example is a group of unusual deposits dated by varve counts at A. The origin of this deposit is not clear, but the deposit provides an important one of several tie points that assisted in the correlation of the stratigraphic sections. Large scale erosional unconformities are conspicuously absent from all outcrops that we examined.
If catastrophic failure of the glacier dam allowed complete subglacial drainage of Iceberg Lake even once prior to A. In three field seasons, while viewing Figure 4.
Cesium concentrations from the uppermost portion of laminated laminated stratigraphy of Iceberg Lake in hundreds of freshly- sediments at Site A. Concentrations are plotted against varve year, the age of each exposed outcrops including the detailed sections shown in Fig. Cesium concentrations in sediment samples from the upper portion of a laminated section at Site A Fig.
Global fallout of cesium from atmospheric nuclear testing began in and peaked in Pennington et al. At site A, Cs concentrations match this pattern; concentrations significantly different than zero first appear in year A.
Number of overlapping laminae shown below diagonal. We verified the ages of deeper strata with radiocarbon dates. Well-dated varve thickness chronologies from seven sites at Iceberg Lake. Each chronology includes measurements from all laminae confidently dated through a combination of varve counting, cross-dating of visually distinctive laminae, and radiogenic techniques. For nine macrofossils found within these sections, thick bars show comparison of calibrated radiocarbon age estimates 2-sigma range given by vertical bars with varve ages horizontal bars.
Radiocarbon data from Table 3. To compare radiocarbon dates on these items with terrestrial storage of dead plant material before transport into stratigraphic ages, we first had to place the laminated strata from the lake. The consistency of these radiogenic dates with our a variety of depths and sites into a common temporal stratigraphic ages confirms that the laminations we are framework. Using visually distinctive layers, we stratigraphi- counting as annual varves each represent a single year of cally correlated varve sections from 7 sites A, B, I, J, K, M, and deposition, and that the cross-dating of cores among multiple N; sections shaded red in Fig.
We found visual matching of sites is generally correct. Even the remaining mismatch, at Site distinctive coloration, textural details, and stratigraphic patterns J cm where the AMS date range is younger than the far more effective than numerical cross-dating. Measurement problems, the possibili- 2with two notable exceptions.
First, sedimentation patterns at ties of occasional missing or multiple laminae in a given year, Site I, which is close to the LIA maximum terminal moraine of and uncertainties about the intervals represented by some Chisma Glacier, appear to have been strongly influenced both anomalous deposits ensure some errors. Based on the by changes in lake level and by changes in the location of the consistency of our stratigraphic ages with radiogenic and nearby Chisma Creek inlet.
These local sedimentation effects other geological evidence, we suggest that errors associated explain the poor correlation of varve thickness at Site I with all with both varve counting and stratigraphic correlation yield a other sites.
We attribute the poor correlation to the very short period of overlap between the Master varve chronology two sites only 51 laminae. We remain confident in the timing of all these sections, however, and collectively they include The varve thickness chronologies presented in Figure 5 and laminations in over 17 m of sediment, with an average tabulated in Supplementary Information Table 1 include all lamination thickness of 4.
Assuming that these well-dated strata. Before doing so, we exclude yr. Foremost among these are all the strata from with our assumptions. For six out of those nine samples, Site I, which as discussed above appear strongly affected by spanning the yr A.
What Are Glacial Varves?
Scattered among the between sections is within the 2-sigma calibrated radiocarbon other well-dated sections are isolated strata that record episodic date of the sample. The case for excluding such deposits from climatologically-oriented varve records has been Table 3 made elsewhere Hardy et al.
Those removed mostly turbidites include many Site Depth Varve 2 sigma range Uncalibrated Sample of the thickest laminae, but sediment structure not thickness cm year yr A. Nor do we adjust the chronologies for changes in a Calibrated 2 sigma ranges rounded to nearest 10 years from Calib 5.
The absence of dilated, low-density 20 M. Master varve thickness chronology from Iceberg Lake. Values are averages of raw measurements from overlapping, stratigraphically correlated sections providing continuous coverage from A. Across all years, mean thickness dashed vertical line is 3.
Thick line shows the same record smoothed with a yr lowpass Butterworth filter.
What are Glacial Varves?
Gray bars on right side of panel indicate numbers of laminae used in construction of master chronology, and opposite them the number of laminae excluded from each year in the plotted chronology due to evidence of turbidites, resuspended sediment, or ice-rafted debris. Since dating errors in the individual chronologies are probably N1 yr, the combined master chronology of varve Discussion thickness Fig.
The most conspicuous trend is an warm spring and summer temperatures, heightened fluvial increase in varve thickness beginning in the late s and discharge, and increased sediment transport capacity in other accelerating in the late s.
This is preceded by a period glacierized and snow-covered basins Hardy et al. At Iceberg Lake, thickness of 3. Throughout, the raw time episodically in response to warming-induced shrinkage of the series is positively skewed, with thickness anomalies above glacier dam Loso et al.
Independent of changes in bulk the mean tending to be greater than those below. This effect sediment delivery to the lake, this shrinkage of the lake M. More telling is the area, and 2 shoreline regression brought high-energy deposi- strong correlation of varve thickness with an updated temper- tional environments associated with creek inlets and deltas ature-sensitive tree-ring thickness chronology from the Wrangell closer to the fixed sampling sites.
We do not know shoreline Mountains Davi et al. This tree-ring chronology is the history prior to the LIA maximum, but we expect that it tracked longest and most directly comparable climate record available temperature trends in a similar fashion: While these constrained, would have amplified rather than diminished the correlation were performed with raw, unfiltered data, for clarity effects of higher frequency, climate-mediated changes in the tree-ring record in Figure 7B has been smoothed with a yr sediment transport capacity: We cannot quantify this relationship at Iceberg Lake.
It is widely recognized that glacier termini respond instrumental records cover only the latter half of the 20th to temperature fluctuations with a characteristic response time century and come from Northway, Slana, and Gulkana, an e. We can, however, document the sensitivity of varve grounds, that glacial sediment yield should reflect lagged thickness at Iceberg Lake to warm season temperatures by changes in erosional efficiency as glaciers grow or shrink, and comparison with other proxy records for temperature Fig.
Other Smoothed versions of the varve thickness chronology and the long-term studies of glacier sediment yield have failed to NH temperature reconstruction of Mann and Jones demonstrate a clear relationship between sediment yields and Figure 7. Comparison of the Iceberg Lake varve chronology panel A, smoothed curve from Figure 6 with other climatological records.
Varve thickness is positively correlated with Mann and Jones' Northern Hemisphere multiproxy temperature reconstruction solid line in panel B, left axis.
The correlation is even stronger with ring-width anomalies from the adjacent Wrangell Mountains updated from Davi et al. Horizontal lines in all panels are arbitrarily placed to emphasize temporal variations.
Details of correlations and smoothing in text.
While complicated varve chronology with a yr low pass filter Fig. In summary, we suggest that the regime. This in turn must reflect warming temperatures and decadal and centennial trends in varve thickness at Iceberg Lake therefore implies that such warm temperatures were not provide a sensitive qualitative record of relative changes in sustained during the MWP.
Assuming that chronologies e. Notably, the minimum varve thickness in the LIA never D. Our chronology does not record the onset of this around A. The Farewell Lake record Hu et reach the bottom of any sectionsbut the timing of this cold al. The timing Finally, varve thicknesses begin climbing around A. In this period of general ley, ; Mann et al. The known history of post-LIA warmth, varve thickness reaches its preth century maximum shoreline lowering at Iceberg Lake Loso et al.
This is a clear period, but we would expect similar exaggeration during other manifestation of the Medieval Warm Period, the regional warm periods like the MWP. That such values are absent from variability of which is poorly documented in Arctic paleotem- the earlier record argues that in no period prior the late 20th perature reconstructions Overpeck et al.
We know of century, in which we have witnessed the progressive and only one other comparable record of southern Alaskan MWP apparently unprecedented collapse of the glacier dam, were temperatures, at Farewell Lake Hu et al.
The Farewell such warm temperatures sustained. Although we cannot quantify the absolute temperatures of the Annual laminations at Iceberg Lake provide a continuous MWP peak, the Iceberg Lake varve record suggests tempera- chronology of suspended sediment deposition between A.
The cross-dated contention that temperatures during the MWP never approached master chronology of varve thicknesses is well correlated with global and regional temperature reconstructions, and we 5 Wiles et al.
While smoothing of Mountains show signs of activity in the centuries following A. Calibrated radiocarbon confidence intervals for all dates associated with these the varve record with a yr low pass filter diminishes the advances extend to A.
International Glaciological Society, Lanzhou, small catchment, and hence glacial sediment production, to China, pp. Introduction to submarine fans and related turbidite climatic forcing. Springer-Verlag, New suggested by the chronology were lowest around A. Holocene coastal glaciation of Alaska. Quaternary Science Reviews 20, —