 |
| Titel |
Annually resolved grain-size distributions in varved sediments using image analysis - application to Paleoclimatology |
| VerfasserIn |
Pierre Francus, François Lapointe, Scott Lamoureux |
| Konferenz |
EGU General Assembly 2013
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077296
|
|
|
|
|
|
| Zusammenfassung |
| Varved sediments are unique archives because they contain continuous and undisturbed
records of past climatic conditions with an internal robust chronology. In many case,
conceptual models for the varve formation can be established linking processes
occurring in the watershed, such as river floods or snow melt, to specific lamina
within the varve structure. However, the physical properties of such layers, including
grain-size, are seldom measured despite their intrinsic value as indicators of hydrological
processes.
This paper reviews the development and improvements of an image analysis methodology
to extract grain-size data from finely laminated sediments. The technique uses thin-sections
from sediment cores, scanning electron microscope images of carefully selected regions of
interest from the thin-sections, and an image analysis routine to extract semi-automatically
grain-size data.
An example from Cape Bounty in the Canadian High Arctic is presented: grain-size data
within each varve was measured for the last 2845 years. Several particle size distribution
indices for each individual facies were calculated and combined to identify each type of
sedimentary facies encountered within the sequence. For instance, high standard deviation
and 98th percentile index values are interpreted as high-energy events such as turbidites and
debris flows.
Moreover, some grain-size indicators from the most recent varves correlate well with
instrumental climate data. For instance, the 98th percentile grain size has a strong correlation
(R2=0.71) with summer rainfall. This kind of relationship allows for the calibration of the
image-analysis generated grain-size data set in terms of hydroclimatic parameters. The
rainfall reconstruction suggests that Cape Bounty recently experienced an unprecedented
increase since ~1920 AD.
These results contrast to other common varve measurements. For instance, varve
thickness is not significantly correlated with the particle size distribution, and is poorly linked
to the instrumental record. Indeed, sediment accumulation can result from the accumulation
of different successive hydroclimatic and geomorphic mechanisms such as spring snowmelt,
rain events and landslides, as well as by changes in lake circulation and stratification.
Therefore, a detailed grain-size obtained using image analysis appears to be a better
approach to reconstruct past hydroclimatic conditions in this clastic sedimentary
setting and holds tremendous potential to improve paleoclimatic reconstructions. |
|
|
|
|
|
|