S. Kempe

An early soda ocean

In analogy to modern soda lakes commonly associated with volcanic regions, it is postulated that the ancient sea had a high alkalinity, a high pH and low Ca and Mg concentrations. The change towards NaCl dominance as observed in the present-day ocean can best be explained by a series of mineral equilibria, crustal differentiation and life processes. In principle hydrothermal leaching of chlorine from the oceanic crust caused this ion to accumulate in the sea while at the same time dissolved carbonates became gradually removed by organisms and carbonates. The switch from a soda to a halite ocean was accomplished ∼1 Ga ago. Critical biochemical events marking the chemical evolution of the Precambrian sea and their implications for the PCO2 in air will be discussed.

Dating Late Glacial abrupt climate changes in the 14,570 yr long continuous varve record of Lake Van, Turkey

In summer 1990, during the third international expedition to Lake Van, eastern Anatolia, 10 sediment cores were retrieved from depths up to 446 m. As reported earlier, the sediments of the lake are finely laminated. The seven cores, recovered up to 30 km apart in the main lake basin, presented sediment sequences which correlate well with respect to ash layers and prominent colour changes, but also lamina for lamina. Here we report on the detailed evaluation of this record, which is varved continuously back to 14,570 yr B.P. (calendar years before 1950 AD). It is independent of 14C calibration, i.e. it is not a floating record, and it is the only detailed varve chronology known from the semi-arid Mediterranean region. Important Late Glacial events, such as the termination of the Oldest and Younger Dryas are clearly recorded in the sediments. Chronozones were defined on the basis of changes of the deposition rate and of chemical composition caused by environmental changes. Analysis of the annual deposition rates revealed abrupt changes within only a few years, declining for example by approximately 30% in the transition period between the Oldest Dryas and the Bolling. In most cases, alterations observed in the sedimentation rates are reflected in changes of the geochemical parameters, such as organic and inorganic carbon, opal, and the major elements Si, Ca, Mg, Al. Our results and palynological studies, performed on material recovered in an earlier expedition, are used to reconstruct palaeoenvironmental conditions. In this study, the termination of the Younger Dryas is dated to 10,920±132 yr B.P. This is younger than the recently published Greenland ice core dates but in accordance with, for example, the central European dendrochronology. We suspect, that higher sediment deposition rates during the cold periods are due to rapid melting and intense wash out of soil, which was fairly loose because of sparse vegetation. This would lead to higher river discharges. Based on the observed increase of the deposition rate in the record, melting of glaciers can only be detected after the termination of the Younger Dryas.

Bacterial diversity and carbonate precipitation in the giant microbialites from the highly alkaline Lake Van, Turkey

Lake Van harbors the largest known microbialites on Earth. The surface of these huge carbonate pinnacles is covered by coccoid cyanobacteria whereas their central axis is occupied by a channel through which neutral, relatively Ca-enriched, groundwater flows into highly alkaline (pH ~9.7) Ca-poor lake water. Previous microscopy observations showed the presence of aragonite globules composed by rounded nanostructures of uncertain origin that resemble similar bodies found in some meteorites. Here, we have carried out fine-scale mineralogical and microbial diversity analyses from surface and internal microbialite samples. Electron transmission microscopy revealed that the nanostructures correspond to rounded aragonite nanoprecipitates. A progressive mineralization of cells by the deposition of nanoprecipitates on their surface was observed from external towards internal microbialite areas. Molecular diversity studies based on 16S rDNA amplification revealed the presence of bacterial lineages affiliated to the Alpha-, Beta- and Gammaproteobacteria, the Cyanobacteria, the Cytophaga-Flexibacter-Bacteroides (CFB) group, the Actinobacteria and the Firmicutes. Cyanobacteria and CFB members were only detected in surface layers. The most abundant and diverse lineages were the Firmicutes (low GC Gram positives). To the exclusion of cyanobacteria, the closest cultivated members to the Lake Van phylotypes were most frequently alkaliphilic and/or heterotrophic bacteria able to degrade complex organics. These heterotrophic bacteria may play a crucial role in the formation of Lake Van microbialites by locally promoting carbonate precipitation.

Interannual variability in particle flux in the southwestern Black Sea

Abstract Vertical particle flux was measured at two sites in the southwestern Black Sea using automated time-series sediment traps over a period of 4.5 years. The particle flux between both sites varied considerably. (1) At site BSC (80 km from shore) the dominant fraction of the annual flux was deposited during short blooms; at site BS (40 km from shore, but still beyond the shelf break), the particle flux was less dominated by short-term blooms. (2) At site BSC, plankton blooms were the dominant cause for removal of suspended lithogenic matter; at site BS, vertical transport of lithogenic matter was linked also to the occurrence of storms and to high discharge periods of local rivers. Upwelling in the southwestern Black Sea may play an important role in triggering plankton blooms.

Climatically induced lake level changes at Lake Van, Turkey, during the Pleistocene/Holocene Transition

Sediment core K10 from Lake Van (eastern Turkey) provides a continuous varve record back to 14,570 calendar years B.P. (before present, 1950), the longest unbroken and non-floating lake varve sequence yet described. The underlying sediment is unvarved and hard. Changes in the aragonite/calcite ratio, the presence of protodolomite and magnesite in certain profile sections, the annual record of the sedimentation rate, the water content of the sediment, the concentrations of organic carbon and opal, and the texture of the sediments from this core provide a record of the lake level history. The new chronology enabled us to redate the old pollen profile [van Zeist and Woldring, 1978a, b] and to establish an accurate timescale for the reconstructed lake level change. Carbon 14 dates show that the highest lake terrace corresponds to high lake level at around 19,000 years B.P. during the Last Glacial, >70 m above its present level. Before 15,000 years B.P. the lake must have been completely dry, marking a reduction of lake level by 500 m in maximum 4000 years. Beginning at 14,600 years B.P. and ending at 12,040 years B.P., the lake level recovered by 250 m to fall again during the next 1400 years. By 10,600 years B.P. the lake began to rise and reached, following another regression between 9000 and 8100 years B.P., the Holocene highstand by about 7500 years B.P., dropping to todays level at about 3000 years B.P.

Alkalinity: The link between anaerobic basins and shallow water carbonates?

Anaerobic ocean basins were common features in earth history. Today, only one large anoxic basin exists, the deep Black Sea (480 000 km3). Massive calcareous deposits are often found in close regional and stratigraphic association with deposits of anoxic basins. The Aptian/Albian and Cenomanian/Turonian anoxic events were, for exarnaple, accompanied by widespread epicontinental chalk and reef limestone deposition. Therefore, one may ask if there is a causal relation between the two facies. In fact, such a connection has been proposed (e.g., [3, 6]). These models describe the general implications of shifting chemoclines and explore consequences of a Ca input, for example, from hydrothermal sources. Here, I present an alternative view which suggests that small changes in the concentration of the total dissolved inorganic carbon (~]CO2) could be responsible. Shifts in ~CO2 affect the supersaturation of CaCO 3 more effectively and therefore faster than the same change in Ca because the Ca concentration in seawater is about ten times that of ~-]CO2. Anaerobic basins are governed by sulfate reduction which mineralizes sinking algal organic matter (i.e., of a Redfield composition [ 12]):

The Hawaiian Archipelago: A Microbial Diversity Hotspot

The Hawaiian Archipelago is a “biodiversity hotspot” where significant endemism among eukaryotes has evolved through geographic isolation and local topography. To address the absence of corresponding region-wide data on Hawaii’s microbiota, we compiled the first 16S SSU rDNA clone libraries and cultivated bacteria from five Hawaiian lakes, an anchialine pool, and the Lō’ihi submarine volcano. These sites offer diverse niches over ~5000 m elevation and ~1150 nautical miles. Each site hosted a distinct prokaryotic community dominated by Bacteria. Cloned sequences fell into 158 groups from 18 Bacteria phyla, while seven were unassigned and two belonged in the Euryarchaeota. Only seven operational taxonomic units (each OTU comprised sequences that shared ≥97% sequence identity) occurred in more than one site. Pure bacterial cultures from all sites fell into 155 groups (each group comprised pure cultures that shared ≥97% 16S SSU rDNA sequence identity) from 10 Bacteria phyla; 15 Proteobacteria and Firmicutes were cultivated from more than one site. One hundred OTUs (60%) and 52 (33.3%) cultures shared <97% 16S SSU rDNA sequence identity with published sequences. Community structure reflected habitat chemistry; most δ-Proteobacteria occurred in anoxic and sulfidic waters of one lake, while β-Proteobacteria were cultivated exclusively from fresh or brackish waters. Novel sequences that affiliate with an Antarctic-specific clade of Deinococci, and Candidate Divisions TM7 and BRC1, extend the geographic ranges of these phyla. Globally and locally remote, as well as physically and chemically diverse, Hawaiian aquatic habitats provide unique niches for the evolution of novel communities and microorganisms.

Genuine modern analogues of Precambrian stromatolites from caldera lakes of Niuafoʻou Island, Tonga

Calcareous or dolomitic, often secondarily silicified, laminated microbial structures known as stromatolites are important keys to reconstruct the chemical and biotic evolution of the early ocean. Most authors assume that cyanobacteria-associated microbialitic structures described from Shark Bay, Western Australia, and Exuma Sound, Bahamas, represent modern marine analogues for Precambrian stromatolites. Although they resemble the Precambrian forms macroscopically, their microstructure and mineralogical composition differ from those characterizing their purported ancient counterparts. Most Precambrian stromatolites are composed of presumably in situ precipitated carbonates, while their assumed modern marine analogues are predominantly products of accretion of grains trapped and bound by microbial, predominantly cyanobacterial, benthic mats and biofilms and only occasionally by their physicochemical activity. It has therefore been suggested that the carbonate chemistry of early Precambrian seawater differed significantly from modern seawater, and that some present-day quasi-marine or non-marine environments supporting growth of calcareous microbialites reflect the hydrochemical conditions controlling the calcification potential of Precambrian microbes better than modern seawater. Here we report the discovery of a non-marine environment sustaining growth of calcareous cyanobacterial microbialites showing macroscopic and microscopic features resembling closely those described from many Precambrian stromatolites.

Satonda Crater Lake, Indonesia: Hydrogeochemistry and biocarbonates

SummaryThe results of detailed hydrochemical and biosedimentological studies of the sea-linked Satonda Crater Lake, Sumbawa Island/Indonesia are presented. They revealed that the mildly alkaline, mid-water stratified and species-poor lake supports growth of cyanobacterial-red algal calcareous reefs comparable with some ancient marine biocarbonates. The chemical and biotic changes during the last 4,000 years of the lake history have been reconstructed. They indicate that the chemistry of the lake evolved from initially fresh water, through highly alkaline to the modern slightly alkaline quasi-marine conditions with corresponding biotic changes. The influence of the 1815 eruption of the nearby located Tambora Volcano on the lake chemistry and resulting lithological and biotic changes is also discussed. The lake proves to be a good model for the recently proposed hypothesis of an early alkaline (soda) ocean.

Chemistry and stromatolites of the sea-linked Satonda crater lake, Indonesia : a recent model for the precambrian sea ?

Abstract The abundance of in situ calcified stromatolites in the Precambrian paleontological record and their absence from modern seas is still an enigma. We report on the discovery of the first recent, in situ calcified stromatolites growing in a sea-linked environment. The stromatolites are produced by coccoid cyanobacteria and started to grow 4000 yr. ago in the Crater Lake of Satonda Island, Indonesia. Compared to seawater, the lake has gained alkalinity and lost Ca. Its pH (8.45) as well as its saturation with respect to calcite is significantly higher than in seawater. The additional alkalinity is provided by biogenic CO2 and subsequent weathering of volcanic silicates. Satonda Crater Lake may provide an analogue to the stromatolite-sustaining Precambrian marine environments which, therefore, may have been more alkaline than present-day seawater. This conclusion is congruent with the recently advanced hypothesis of an early “soda ocean”.