Scott F. Lamoureux

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.

Embedding unfrozen lake sediments for thin section preparation

A method for subsampling and embedding unfrozen sediments is described. The method requires commonly available equipment and can be implemented efficiently and cost-effectively. Thin sections prepared from the embedded unfrozen sediments preserve very fine structural detail, without interruption due to ice-crystal casts. The method provides a viable alternative to subsampling methods requiring either frozen sediments or specialized equipment.

Five centuries of interannual sediment yield and rainfall‐induced erosion in the Canadian High Arctic recorded in lacustrine varves

A 487-year sediment yield record based on the varves from Nicolay Lake, Nunavut, Canada, is used to investigate long-term yield variability. A general extreme value model, estimated with probability-weighted moments, was used to identify the magnitude and timing of the low-probability events. Exceptional sediment yields in 1951 and 1962 coincided with the two largest rainfalls on record. Smaller multiday rainfalls were also recorded in the varves as subannual rhythmites, although rhythmites were not generated by rainfall events that occurred during the nival flood or after prolonged warm weather. By accounting for probable soil moisture conditions and the timing of nival floods compared to major rainfall, rhythmites in all but 3 years of the sediment record can be explained by storms exceeding ∼13 mm total. Generally, more frequent extremes and increased variance in yield occurred during the 17th and 19th centuries, likely due to increased occurrences of cool, wet synoptic types during the coldest periods of the Little Ice Age.

A late Holocene varved sediment record of environmental change from northern Ellesmere Island, Canada

A composite record of varve sedimentation is presented from high arctic meromictic Lake C2. The combination of a short runoff and sediment transport season with the strong density stratification of the lake lead to the formation of annual sediment couplets. This conclusion was confirmed by 210Pb determinations. High intra-lake correlation of the varves allowed the construction of a composite record of varve sedimentation from overlapping segments of multiple sediment cores. Cross-dating between core segments isolated counting errors in individual cores, that could be attributed to minor sediment disturbances and vague structures. Resolving counting errors by cross-dating reduced the chronological error of the composite series to an estimated ±57 years.The Lake C2 series is the first non-ice cap, high resolution late-Holocene environmental record from the Canadian high arctic. The composite varve series compares favorably with other high resolution proxies from the arctic, in particular with the ice core records from Devon Island and Camp Century, Greenland. A general correspondence between the varve record and other North American proxies for the little Ice Age period (1400–1900 AD) suggests that the Lake C2 record is sensitive to large-scale synoptic changes.

Varve Chronology Techniques

Growing interest in past environmental conditions and the need to obtain a long-term perspective on environmental change is leading to increasing use of high-resolution lake sediment records to obtain the clearest indication of past changes. Varved sediments are particularly important for high-resolution studies because they provide a clear, simple means of identifying one year of deposition within along sedimentary sequence. The simple, consistent time increment found in varved sequences provides an unparalleled framework for reconstructing and documenting detailed changes in long term environmental conditions, ranging from hundreds to more than ten thousand years into the past. Additionally, varves can be composed of a variety of materials (biological, chemical, mineral) and are found in a wide range of environments. Therefore, varve-based sedimentary records have applications in a diverse range of paleoenvironmental research fields and study locations. Varves have played a prominent role in numerous geological studies in northern Europe and North America that began in the latter half of the century. Although varved clay deposits were documented in Sweden as early as 1855, it was the linkage between the varves and glacial theories that led the Swedish geologist Gerald De Geer to suggest that varves represented a one-year cycle of deposition. This was largely based on the similarity between the structure of varves and tree rings (Brunnberg, 1995). De Geer suggested that the silt layer represented deposition during the melt season and the clay layer was produced during the cold season under lake ice. By making use of the extensive outcrops of varves found in southern Sweden, De Geer (1912) pioneered the use of varves as a chronological tool to estimate the age of ice retreat. In this and subsequent work, De Geer and other workers measured varve sequences to construct varve thickness chronologies from various locations. These chronologies were correlated to develop a single varve time scale of the postglacial period. This work provided a foundation for future varve work by linking together isolated “floating” chronologies using distinctive marker varves found at multiple sites (De Geer 1912, 1934). Similar to the process routinely used in dendrochronology (Fritts, 1976),

Fluvial Impact of Extensive Active Layer Detachments/ Cape Bounty, Melville Island, Canada

Abstract Exceptional and persistent warm temperatures recorded during July 2007 at Cape Bounty, Melville Island, Canada (74°54′N, 109°35′W), resulted in rapid and deep active layer formation. The thickened active layer, together with up to 10.8 mm of rainfall in late July, resulted in widespread active layer detachments across the West watershed during 23–31 July. Mapping indicates that approximately 1.9% of the watershed was directly impacted by disturbances. By contrast, only two small detachments occurred in the adjacent East watershed. The immediate fluvial impact of the detachments was primarily in the form of abrupt, short-lived rises in river turbidity, along with a more gradual increase in discharge and overall turbidity. Sediment transport pulses resulted from the hydrological connection of major detachment slides, most of which were upslope from the main channel. The largest detachment dammed the river over a length of 200 m, and resulted in an upstream pond and prolonged increased sediment transport. In total, the increased sediment transport during the last week of July amounted to an estimated 44.3 Mg, or 18% of the seasonal yield. While the detachments had an immediate and substantial impact on river conditions, erosion of unstable material is likely to have a sustained impact on watershed fluxes in future years.

Climatic Controls on Streamflow and Suspended Sediment Transport in Three Large Middle Arctic Catchments, Boothia Peninsula, Nunavut, Canada

Abstract Streamflow, suspended sediment transport, and meteorological variables were measured during two field seasons in order to determine the climatic controls on daily discharge and suspended sediment load for three large middle arctic catchments. Substantive suspended sediment transfer only occurred during the short-lived nival peak, and the duration of the peak appears to be broadly scaled with interannual catchment snow water equivalence (SWE). Thermal energy was critical in generating streamflow and suspended sediment transfer, but only until watershed snowpack had been exhausted. Thus, total annual suspended sediment load in this environment is ultimately a function of total discharge through SWE rather than melt energy. Specific sediment yields were some of the lowest recorded in the arctic, ranging between 0.2 and 1.9 t km−2·a−1. This study demonstrates the sensitivity middle arctic watersheds to both winter snowpack and spring thermal conditions and provides a basis for understanding the hydrological impact of future climate changes.

A new method for visualizing sediment particle size distributions

Conventional uses of sediment grain size data as a proxy for paleoenvironmental processes are usually restricted to simple statistical descriptions of the particle size distribution (PSD) (e.g., mean, median and mode). These descriptions provide a generalized indication of down-core changes in depositional conditions and processes, but are not sensitive to non-normal or polymodal distributions. In contrast, surface plots of grain size data allow qualitative interpretation of the characteristics of the entire PSD and thus can provide important insights into depositional processes and changing environmental conditions. This method is especially useful for distinguishing multiple sedimentary processes, which can appear as additional modes within the PSD. In general, this approach can be used on sediments from most depositional environments and at any spatial or temporal resolution. In combination with conventional summary statistics, PSD surface plots increase the potential utility of grain size as a paleoenvironmental proxy for identifying changes in clastic and organic depositional processes in lake sequences.

Arctic Permafrost Active Layer Detachments Stimulate Microbial Activity and Degradation of Soil Organic Matter

Large quantities of soil organic carbon in Arctic permafrost zones are becoming increasingly unstable due to a warming climate. High temperatures and substantial rainfall in July 2007 in the Canadian High Arctic resulted in permafrost active layer detachments (ALDs) that redistributed soils throughout a small watershed in Nunavut, Canada. Molecular biomarkers and NMR spectroscopy were used to measure how ALDs may lead to microbial activity and decomposition of previously unavailable soil organic matter (SOM). Increased concentrations of extracted bacterial phospholipid fatty acids (PLFAs) and large contributions from bacterial protein/peptides in the NMR spectra at recent ALDs suggest increased microbial activity. PLFAs were appreciably depleted in a soil sample where ALDs occurred prior to 2003. However an enrichment of bacterial derived peptidoglycan was observed by (1)H-(13)C heteronuclear multiple quantum coherence (HMQC) and (1)H diffusion edited (DE) NMR and enhanced SOM degradation was observed by (13)C solid-state NMR. These data suggest that a previous rise in microbial activity, as is currently underway at the recent ALD site, led to degradation and depletion of labile SOM components. Therefore, this study indicates that ALDs may amplify climate change due to the release of labile SOM substrates from thawing High Arctic permafrost.

Climate Change and Mercury Accumulation in Canadian High and Subarctic Lakes

Mercury (Hg) profiles were compared to profiles of climate indicators including microfossil remains and algal-derived or S2 carbon (C) in dated sediment cores from 14 lakes spanning latitudinal and longitudinal gradients across the Canadian high and subarctic. Hg fluxes increased postindustrialization (post-∼1850) in 11 of these lakes (postindustrialization Hg fluxes (ΔHgF(F)) = 2-24 μg m(-2) y(-1)). Correction of HgF(F) for catchment contributions demonstrated that Hg deposition originating from catchment-independent factors, such as atmospheric deposition, increased since industrialization in all 14 lakes. Several of these lakes also showed postindustrial shifts in algal assemblages consistent with climate-induced changes. Eleven lakes showed post-1850s increases in S2F(F), suggesting that lake primary productivity has recently increased in the majority of our sites (ΔS2F(F) = 0.1-4 g m(-2) y(-1)). Other studies have interpreted significant relationships between Hg:S2 concentrations in Arctic sediment as support for the algal scavenging hypothesis, which postulates that Hg fluxes to Arctic sediments are largely driven by S2. However, in six of our lakes we observed no Hg:S2 relationship, and in one lake a significant negative Hg:S2 relationship was observed due to increased Hg and decreased S2 C deposition during the postindustrialization period. In six of the seven lakes where a significant positive Hg:S2 relationship was observed, algal assemblages either did not change through time or the timing of the shifts did not correspond to changes in Hg deposition. Our results demonstrate that, although Arctic lakes are experiencing a myriad of changes, including increased Hg and S2 deposition or changing algal assemblages, increased lake primary productivity does not appear to be driving changes in Hg fluxes to sediments.