Pierre Francus

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Climate-driven ecosystem succession in the Sahara: The past 6000 years

Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a “green Sahara” to the present hyperarid desert. Our continuous 6000-year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site-specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large-scale dust mobilization from 4300 calendar years before the present (cal yr B.P.). Todays desert ecosystem and regional wind regime were established around 2700 cal yr B.P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate.

An image-analysis technique to measure grain-size variation in thin sections of soft clastic sediments

This paper presents a simple and fast image-analysis technique to measure grain-size variation in thin sections of soft elastic sediments. Optical and backscattered electron microscope photographs are digitised from thin sections, cut by freeze-drying to avoid disturbing the sediments. Processing of the 256 grey-scale images produces binary (black and white) images, where white pixels represent the clay-rich sedimentary matrix and black pixels represent elastic grains. Data measured include grain surface, grain perimeter, and centre of gravity. Two indices are defined to quantify grain-size: D-0 = equivalent disk diameter; and P% = phase percentages. The technique is applied to silt and clay laminations attributed to the D-division of a distal turbidite from Lake Baikal. The data obtained using image analysis are similar to the conventional descriptions of such deposits. Indices calculated on very small samples, e.g. on a single lamina, allow for greater precision in interpretation of sedimentary processes. Compared to photomicrographs, backscattered electron microscope images have better qualities and allow to quantify variations in the grain-size along a sediment core. The technique produces objective and quantified grain-size observations in less time than conventional petrographic methods

Rapid Lacustrine Response to Recent High Arctic Warming: A Diatom Record from Sawtooth Lake, Ellesmere Island, Nunavut

Abstract Diatoms from Sawtooth Lake (79°20′N, 81°51′W) on the Fosheim Peninsula in Central Ellesmere Island, Canada were analyzed to assess the temporal extent and magnitude of climatic change in the High Arctic during the late Holocene. Diatom results from the sediment cores show an absence of diatoms throughout the last ∼2.5 ka (4.6 m) until the 1920s. However, ca. 1926 (5.3-cm depth), a rapid colonization of diatoms in the lake occurred. Within the uppermost section of the core (∼1920 to ∼1997), the diatom flora shift from a small Fragilaria-dominated assemblage to a more diverse assemblage that is dominated by large planktonic taxa (e.g., Cyclotella bodanica) and large raphid benthic species. The postglacial nature of this assemblage suggests a decrease in ice cover and a concomitant increase in light and nutrient availability for diatom growth over the last ∼75 yr. Of particular significance is this absence of diatoms prior to the ∼1920s, which indicates that environmental conditions of the last ∼75 yr are unlike any of the previous ∼2500 yr.

Changes in erosion patterns during the Holocene in a currently treeless subalpine catchment inferred from lake sediment geochemistry (Lake Anterne, 2063 m a.s.l., NW French Alps): The role of climate and human activities

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

The high sensitivity of the palynological record in the Vico maar lacustrine sequence (Latium, Italy) highlights the climatic gradient through Europe for the last 90 ka

The palynological study of the Vice maar lacustrine sequence (Italy) is realized from the correlation of three drillholes. The time span covers a period from the end of Oxygen Isotope Substage 5b till the beginning of Stage 1 (from ca. 90 to ca. 10 ka BP). At the base of the pollen diagram (pollen zone 0), a Younger Dryas-like fluctuation is observed. After pollen zone 1, which is a well developed forest period (Substage 5a), the forests of pollen zones 3 and 5 correspond to extensive climate improvements. These last two zones are correlated to the Ognon complex of the Grande Pile (France) and to interstadial periods 20 and 19 defined in the GRIP ice core (Greenland) (72.6-66.2 ka) before the Stage 5/4 transition. These two interstadial periods are global events found in the Greenland and Antarctica ice sheets, as well as in North Atlantic marine cores. In other pollen records from north of the Alps, changes have been recorded but they are less pronounced than in Vico. Pollen zone 7 (Stage 3) shows ca. 7 warm/humid fluctuations, that might have a link to the Dansgaard-Oeschger events (Greenland). During zone 9, the Lateglacial interstadial is recorded as well as the Younger Dryas event. The localization of Vice near plant refugia and far from the ice cap enables its pollen diagram to record abrupt and intense response to climate changes, possibly through a link to the Atlantic Ocean via westerly winds

Sub-sampling unconsolidated sediments: A solution for the preparation of undistrubed thin-sections from clay‐rich sediments

Using the principles of electro‐osmosis, a sub‐sampling technique for clay-rich sediments is introduced that produces undisturbed thin‐sections from sediment cores. The fragile sample remains in the same disposable box throughout the process, preventing collapse of its structure during sub‐sampling, manipulation and impregnation, as well as other potential problems.

Holocene dynamics of the Arctic's largest ice shelf

Ice shelves in the Arctic lost more than 90% of their total surface area during the 20th century and are continuing to disintegrate rapidly. The significance of these changes, however, is obscured by the poorly constrained ontogeny of Arctic ice shelves. Here we use the sedimentary record behind the largest remaining ice shelf in the Arctic, the Ward Hunt Ice Shelf (Ellesmere Island, Canada), to establish a long-term context in which to evaluate recent ice-shelf deterioration. Multiproxy analysis of sediment cores revealed pronounced biological and geochemical changes in Disraeli Fiord in response to the formation of the Ward Hunt Ice Shelf and its fluctuations through time. Our results show that the ice shelf was absent during the early Holocene and formed 4,000 years ago in response to climate cooling. Paleoecological data then indicate that the Ward Hunt Ice Shelf remained stable for almost three millennia before a major fracturing event that occurred ∼1,400 years ago. After reformation ∼800 years ago, freshwater was a constant feature of Disraeli Fiord until the catastrophic drainage of its epishelf lake in the early 21st century.

An Extreme Sediment Transfer Event in a Canadian High Arctic Stream

Abstract Two large suspended sediment concentration (SSC) pulses were recorded in 1998 in a small snowmelt-fed stream on Ellesmere Island in the Canadian High Arctic. The largest pulse occurred from 7 to 8 July, when 32% of the monitored seasonal sediment transport occurred in only four hours. SSC reached 83,760 mg L−1, exceeding all previously recorded values from high arctic glacially-fed and snowmelt-fed rivers by more than one order of magnitude. The event occurred after the majority of snow in the watershed had melted, and was preceded by a long period of relatively high air temperature, and a small rainfall event on 7 July. We consider the most likely cause of the event to be a rapid mass movement.

An algorithm to aid varve counting and measurement from thin-sections

An algorithm has been implemented to count and measure laminations from digital images of petrographic thin-sections. Boundaries and limits of laminations are first marked manually using imaging software. These marks are then exported to a workable format output file. Using the output file, the algorithm extracts the relevant information for lamination counts and thickness measurements along one or more parallel vertical lines. Compared to manual counts, the use of the algorithm improves the accuracy of measurements and speeds up the process. The method requires tools commonly available in most laboratories, i.e., a digital camera or a flat bed scanner with transparency capability and a desktop computer with 64 MB of RAM. From the algorithm, two utilities are implemented in Visual Basic for Applications and Fortran to be used on both Apple Macintosh and a PC-compatible, with Adobe® Photoshop®.