David B. Ryves

Catastrophic Drought in the Afro-Asian Monsoon Region During Heinrich Event 1

An extreme megadrought occurred in the Afro-Asian monsoon region during an iceberg melting episode 50,000 years ago. Between 15,000 and 18,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here, we demonstrate that the height of this stadial, about 16,000 to 17,000 years ago (Heinrich event 1), coincided with one of the most extreme and widespread megadroughts of the past 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the intertropical convergence zone, but the broad geographic range of the Heinrich event 1 megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling.

Experimental diatom dissolution and the quantification of microfossil preservation in sediments

Abstract Four laboratory experiments on fresh, modern diatoms collected from lakes in the Northern Great Plains of North America were carried out to assess the effects of dissolution on diatom abundance and composition. Marked differences in mean dissolution susceptibility exist between species, despite sometimes significant intra-specific variation between heterovalves. Twenty-four taxa were ranked according to susceptibility to dissolution using an exponential decay model of valve abundance. This dissolution ranking was used to derive two weighted indices of sample preservation. A third index (F) was based on a simple binary classification of valve morphology into dissolved and pristine categories, as distinguished by light microscopy (LM). When compared against rank indices and a measure of species diversity, this diatom dissolution index was found to be the best predictor of the progress of dissolution as estimated by total valve abundance or biogenic silica (BiSi) loss. Strong empirical relationships between F index values and diatom abundance (r2=0.84, n=32) and BiSi (r2=0.89, n=32) were developed and applied to a diatom sequence from a short core from Devils Lake, North Dakota, and compared to diatom-inferred and observed salinity at this site. The F index is a simple, effective diagnostic tool to assess important aspects of diatom preservation. The index can provide insight into Si cycling and record changes in conditions pertinent to diatom dissolution, and has a role in validation of transfer functions or other inferences derived from compositional data.

Climate Versus In-Lake Processes as Controls on the Development of Community Structure in a Low-Arctic Lake (South-West Greenland)

The dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A 14C and 210Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic–inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (δ13C, δ18O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18–32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12–17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.

Holocene records of effective precipitation in West Greenland

Holocene changes in effective precipitation (precipitation-evaporation; P-E) were reconstructed for the Søndre Strømfjord region of southwest Greenland using the sediment records of two neighbouring closed-basin ‘saline’ lakes. Past lakewater conductivities (a proxy for P-E balance) were estimated using a diatom-inferred conductivity model. Broadly similar changes in both lake records corroborate the use of DI conductivity as a regional climate proxy. An increasing DI conductivity trend through the Holocene indicated net negative or balanced P-E ratios in this region, even during most of the Neoglacial period (post-4000 yr BP). This contrasts with other regions of West Greenland, where conditions became more humid in the Neoglacial period. The reconstructions presented here therefore suggest a high degree of spatial variability in the P-E balance, likely caused by region-specific orographic features. While interpret ation of the mid-Holocene P-E balance was limited by dissolution of the diatom record, other sediment indicators suggest a period of extremely negative effective precipitation (~7000–5600 yr BP), followed by a period of positive effective precipitation lasting until 4700 yr BP. This contrasts markedly with the later Holocene, after c. 4000 yr BP, when high-frequency oscillations in DI conductivity probably reflect short-term climatic variations (amplified by in-lake processes connected with meromixis) but no long-term trends in the P-E balance.

Dominant Factors Controlling Variability in the Ionic Composition of West Greenland Lakes

Over 80 lakes were sampled between the ice margin and the coast in West Greenland between 66 and 67°N and analyzed for their pH, alkalinity, conductivity, and major ions. Most of the lakes (67%) are dilute, circumneutral, and have conductivity values <150 μS cm−1. There are, however, also saline lakes (2000–4000 μS cm−1; NaHCO3/CO3, and Mg HCO3/CO3-dominated), mainly around the head of the fjord. The main control on lake chemistry in this area is evaporation as indicated by the fossil shorelines surrounding the saline lakes. The high concentrations of Na indicate extensive inputs from either weathering or from eoliantransported silts (loess) derived from the sand outwash plains at the ice sheet margin. The pattern of cation dominance in the freshwater lakes is Ca > Mg > Na > K. Variability in the water chemistry and the dominant controls on water chemistry were examined using PCA and RDA, respectively. There is a clear gradient in lake chemistry from west to east. This trend reflects the split between the dilute coastal lakes (with higher %C1) and the saline lakes close to the head of the fjord. Altitude is also significant and probably reflects reduced weathering rates at upland sites (~500 m) with limited vegetation cover. A number of lakes had high SO4 concentrations (maximum >10,000 μeq L−1) and were clearly influenced by local geology.

Diel surface temperature range scales with lake size

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer diel ranges in their near-surface temperatures of between 4 and 7°C. Large diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored.

Identifying sedimentation patterns in Lake Baikal using whole core and surface scanning magnetic suceptibility

Forty seven ca. 1 m sediment cores were collected from Lake Baikal during a summer cruise in 1996 and analysed for whole-core susceptibility. Fifteen of these cores were further analysed using a new prototype surface scanning sensor on board the ship R.V. Vereshchagin. The main purpose of this paper is to show that the measurement of Lake Baikal short cores using two susceptibility sensors gives valuable field data and can be used as a tool for identifying undisturbed sediment sequences. Four coring transects were sampled to identify sedimentation patterns reaching from the shelves and sub-basins of the near lake shore and across mainly the northern basin of Lake Baikal (water depth ca. 1500 m). Also in the sub-basins and in the southern basin other groups of cores were taken. One of the main sediment features of interest is that of turbidite sedimentation. Whole core magnetic susceptibility traces are used to identify turbidite ‘fingerprints’ and correlate them between cores along the transects. The results from the two magnetic susceptibility sensors the whole-core sensor and the new prototype surface scanning sensor, both giving volume Kappa values, are compared and are found to be significantly correlated given the difference in resolution. The whole-core sensor gives a smoothed equivalent to a ‘moving average’ curve of magnetic susceptibility while the surface scanner can give fine resolution (ca. 2 mm) results picking out fine peaks with Kappa values of between 150 to 650.The results show that most turbidite sedimentation can be clearly identified; they give a specific magnetic susceptibility ‘fingerprint’ with larger Kappa values (up to 120) at the base of the turbidite corresponding with the settling of coarser sandy sediments and a steady and gradual decline in values to about 15 at the top of the turbidite where the fines settle incorporating the normal diatomaceous sedimentation. The main control on the magnetic susceptibility of the turbidite sediments is the concentration of ferrimagnetic minerals in different particle size fractions. The turbidites can be correlated between many of the cores collected along the transects but it must be noted that these correlations are partly speculative and will be confirmed with future dating, diatom analysis and geochemistry. Other very fine peaks of less than 5 mm in width identified using the surface scanning sensor may indicate concentrations of ferrimagnetic minerals, namely greigite, formed during the reduction phase.

Evidence for short-lived oscillations in the biological records from the sediments of Lago Albano (Central Italy) spanning the period ca. 28 to 17 k yr BP

We report the results of analyses of pigments (derived from algae and photosynthetic bacteria), diatoms and invertebrate fossil remains (ostracods, cladocerans, chironomids) in two late Pleistocene sediment cores from Lago Albano, a crater lake in Central Italy. The record contains evidence for oscillations in lake biota throughout the period ca. 28 to 17 k yr BP. The earliest of these are contained in the basal 3.5 m of light olive-gray and yellowish-gray spotted muds sampled in core PALB 94-1E from 70 m water depth. The later oscillations are best represented in the more extended sediment sequence recovered from a second core site, PALB 94-6B, in 30 m water depth. The sediments at site 1E, containing the earlier oscillations (ca. 28-24 k yr BP), predate any sedimentation at the shallower site, from which we infer an initially low lake level rising to permit sediment accumulation at site 6B from ca. 24 k yr onwards. At site 6B, massive silts rich in moss remains are interbedded with laminated silts and carbonates. These sediments span the period ca. 24 to 17 k yr and are interpreted as representing, respectively, times of shallow water alternating with higher lake stands, when the lake was stratified and bottom water was stagnant. A range of mutually independent chronological constraints on the frequency and duration of the oscillations recorded in the lake biota indicate that they were aperiodic and occurred on millennial to century timescales. We interpret them as responses to climate forcing through its impact on lake levels and changing aquatic productivity. The time span they occupy, their frequency and their duration suggest that at least some of these changes may parallel both the Dansgaard-Oeschger events recorded in Greenland Ice Cores and the contemporary oscillations in North Atlantic circulation documented in marine sediment cores.

Cladoceran stratigraphy in two shallow brackish lakes with special reference to changes in salinity, macrophyte abundance and fish predation

Sub-fossils of Cladocera and Foraminifera were used to reconstruct changes since 1870 in the trophic dynamics of two brackish lakes, Glombak and Han Vejle, located in the Vejlerne nature reserve, Denmark, a site of international conservation importance. After creation of the lakes in the mid-1870s following land reclamation, the two lakes have developed quite differently; today Glombak is turbid, while Han Vejle is clear. In both lakes, stratigraphic changes in the assemblages of foraminifers and cladocerans indicate an abrupt shift from marine to brackish conditions at the end of the 19th century, coinciding with land reclamation. However, the composition of the fossil invertebrate assemblages in the 20th century implies differences in the exposure to salinity, in fish predation and in habitat diversity. In Glombak, the cladoceran record suggests relatively saline conditions in the first quarter of the last century and high macrophyte abundance followed by lower salinities and subsequently a major decrease in macrophyte abundance and an increase in fish predation during the past ca. 40 years. By contrast, in Han Vejle low salinity, high abundance of macrophytes and only minor changes in fish predation seem to have prevailed throughout most of the 20th century. The results are consistent with recent contemporary data, the few historical records, as well as with trends in the records of diatoms and macrofossils. This study highlights the potential of using crustacean remains as indicators of long-term changes in the trophic dynamics of brackish lakes.

A late glacial and holocene record of biological and environmental changes from the crater Lake Albano, Central Italy: An interdisciplinary european project (PALICLAS)

This paper reports the results of biological analyses (pigments, diatoms, chrysophyte cysts, cladocerans, chironomids and ostracods) of a ca. 14 m-long sediment core recovered from Lake Albano (Central Italy) in the course of the EU-funded project PALICLAS (PALaeoenvironmental analysis of Italian Crater Lake and Adriatic Sediments).A reconstruction of the environmental evolution and ecosystem response of Lake Albano during the last ca. 30 kyr was possible. Additional information on lake level oscillation is obtained from benthic and planktonic palaeocommunities. Several oscillations in the productivity and the level of the lake were detected in the oldest sediment layers (from ca. 30 kyr BP to ca. 17 kyr BP), followed by a long (ca. 5 kyr BP) period of low productivity in which cold, holomictic conditions prevailed. A period of high biological activity and, probably, meromictic conditions during the early-mid Holocene was detected. A clear impact of human activities in the catchment was found at ca. 4 kyr BP in the form of increased erosion, associated with a decline in the abundance of biological remains. Further signs of human impact on the lake ecosystem are recorded during the Roman period. Although large-scale environmental changes (e.g. regional climate changes) caused many of the observed biological changes, human activities were important during the mid-late Holocene.