Juha Karhu

University of Helsinki, Geosciences and Geography, Faculty Member

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The Paleoproterozoic Francevillian succession of Gabon and the Lomagundi-Jatuli event: COMMENT

Geology, Jul 1, 2021

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Paljonko maapallo painaa?: Lasten tiedekysymykset

The effect of water pre-treatment with poly-aluminiumchloride on the decomposition of dissolved organic matter in managed aquifer recharge

Bulletin of the Geological Society of Finland, 2020

This study examines the effect of poly-aluminium chloride (PACl) pre-treatment on the biological ... more This study examines the effect of poly-aluminium chloride (PACl) pre-treatment on the biological purification process taking place during managed aquifer recharge (MAR). PACl treatment is used in waste and surface water treatment to remove organic material. PACl can decrease the organic carbon contents of water significantly. However, aluminium is toxic to microbes, which may be detrimental when the PACl treatment is followed by infiltration into an aquifer (MAR). In this study the effect of pre-treatment with PACl on the water purification process in MAR was examined, by using sediment from a MAR site in four test columns. Two columns were filled with PACl-treated water, and the remaining ones with river water without PACl. The residence time of water in the columns was raised from direct sampling gradually to 64 days. Among the parameters monitored were the pH of the water, microbial activity and the decomposition of dissolved organic carbon (DOC). The results showed active decomp...

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Minimal biomass deposition in banded iron formations inferred from organic matter and clay relationships

Nature Communications, 2019

The cycling of iron and organic matter (OM) is thought to have been a major biogeochemical cycle ... more The cycling of iron and organic matter (OM) is thought to have been a major biogeochemical cycle in the early ferruginous oceans which contributed to the deposition of banded iron formations (BIF). However, BIF are deficient in OM, which is postulated to be the result of near-complete oxidation of OM during iron reduction. We test this idea by documenting the prevalence of OM in clays within BIF and clays in shales associated with BIF. We find in shales >80% of OM occurs in clays, but <1% occurs in clays within BIF. Instead, in BIF OM occurs with 13C-depleted carbonate and apatite, implying OM oxidation occurred. Conversely, BIF which possess primary clays would be expected to preserve OM in clays, yet this is not seen. This implies OM deposition in silicate-bearing BIF would have been minimal, this consequently stifled iron-cycling and primary productivity through the retention of nutrients in the sediments.

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Lomagundi Carbon-Isotope Excursion on the Amazonian Craton

Geological Society of America Abstracts with Programs, 2017

Carbon Isotopes in the Solar System

Isotopic signatures of precent-day calcite and pyrite in low-temperature crystalline bedrock, Olkiluoto, SW Finland

Applied Geochemistry

Reply to comment on “Bekker, A., Krapež, B., Karhu, J.A., 2020. Correlation of the stratigraphic cover of the Pilbara and Kaapvaal cratons recording the lead up to Paleoproterozoic Icehouse and the GOE. Earth-Science Reviews, 211, 103,389” by Pascal Philippot, Bryan A. Killingsworth, Jean-Louis P...

Earth-Science Reviews, 2021

Thriving or surviving? The isotopic record of the Wrangel Island woolly mammoth population

Quaternary Science Reviews, 2019

The world's last population of woolly mammoths (Mammuthus primigenius) lived on Wrangel Island pe... more The world's last population of woolly mammoths (Mammuthus primigenius) lived on Wrangel Island persisting well into the Holocene, going extinct at ca. 4000 cal BP. According to the frequency of radiocarbon dated mammoth remains from the island, the extinction appears fairly abrupt. This study investigates the ecology of the Wrangel Island mammoth population by means of carbon, nitrogen and sulfur isotope analyses. We report new isotope data on 77 radiocarbon dated mammoth specimens from Wrangel Island and Siberia, and evaluate them in relation to previously published isotope data for Pleistocene mammoths from Beringia and lower latitude Eurasia, and the other insular Holocene mammoth population from St. Paul Island. Contrary to prior suggestions of gradual habitat deterioration, the nitrogen isotope values of the Wrangel Island mammoths do not support a decline in forage quality/ quantity, and are in fact very similar to their north Beringian forebears right to the end. However, compared to Siberian mammoths, those from Wrangel Island show a difference in their energy economy as judged by the carbon isotope values of structural carbonate, possibly representing a lower need of adaptive strategies for survival in extreme cold. Increased mid-Holocene weathering of rock formations in the central mountains is suggested by sulfur isotope values. Scenarios related to water quality problems stemming from increased weathering, and a possibility of a catastrophic starvation event as a cause of, or contributing factor in their demise are discussed.

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Geochemical and flow modelling as tools in monitoring managed aquifer recharge

Applied Geochemistry, 2016

Abstract Due to a growing world population and the effects of anthropogenic climate change, acces... more Abstract Due to a growing world population and the effects of anthropogenic climate change, access to clean water is a growing global concern. Managed aquifer recharge (MAR) is a method that can help society's response to this increasing demand for pure water. In MAR, the groundwater resources are replenished and the quality of the recharged surface water is improved through effects such as the removal of organic matter. This removal occurs through mechanisms such as microbial decomposition, which can be monitored by studying the isotopic composition of dissolved inorganic carbon (DIC). Nevertheless, the monitoring can be difficult when there are other factors, like dissolving calcite, affecting the isotopic composition of DIC. The aims of this study were to establish a method for monitoring the decomposition of organic matter (dissolved organic carbon – DOC) in cases where calcite dissolution adds another component to the DIC pool, and to use this method to monitor the beginning and amount of DOC decomposition on a MAR site at Virttaankangas, southwestern Finland. To achieve this, we calculated the mean residence times of infiltrated water in the aquifer and the fractions of this water reaching observation wells. We conducted geochemical modelling, using PHREEQC, to estimate the amount of DOC decomposition and the mineral reactions affecting the quality of the water.

A short-term, post-Lomagundi positive C isotope excursion atc.2.03 Ga recorded by the Wooly Dolomite, Western Australia

Journal of the Geological Society, 2016

It is generally accepted that carbon isotope variations in seawater were muted between c. 2.06 Ga... more It is generally accepted that carbon isotope variations in seawater were muted between c. 2.06 Ga, after the end of the Lomagundi carbon isotope excursion (LCIE), and c. 1.3 Ga. Evidence is presented here that c. 30 myr after the end of the LCIE, the biogeochemical cycle of carbon experienced a short-term (c. 2 myr), high-amplitude (up to +8.4‰ V-PDB) perturbation, recorded in the Horseshoe rift basin, Western Australia. The basin was initiated at c. 2.03 Ga with deposition of fluvial and shallow-marine sandstones, followed by the eruption of flood basalt, and culminated with the deposition of platform carbonates, and accompanying volcaniclastic and siliciclastic sediments (Wooly Dolomite). The Horseshoe rift basin during deposition of the Wooly Dolomite was fault-compartmentalized but connected to an ocean. Six depositional sequences make up the Wooly Dolomite. Sequence 1 records establishment of a carbonate platform conformably on basalt and coevally with volcaniclastic sedimentation. All other sequences have dominant carbonate-platform deposits and are unconformity-bounded. Sequence 3 contains a c. 57 m thick section with 13C-enriched carbonates bracketed between carbonates with close to 0‰ carbon isotope values. Further high-resolution chemostratigraphic studies may reveal a more complex pattern of carbon isotope variations during the ‘boring billion years’, but without precise geochronology similar short-term carbon isotope excursions in carbonate successions could be incorrectly correlated to the LCIE. Supplementary material: Table 2 including chemical and isotopic data, sample locations and their position in Figure 8 is available at https://doi.org/10.6084/m9.figshare.c.2868055.

Paleoproterozoic record of biogeochemical events and ice ages

Carbon isotope record for the onset of the Lomagundi carbon isotope excursion in the Great Lakes area, North America

Precambrian Research, 2006

Carbonate units of the Chocolay Group in the Lake Superior area, USA and the Gordon Lake Formatio... more Carbonate units of the Chocolay Group in the Lake Superior area, USA and the Gordon Lake Formation of the correlative upper Huronian Supergroup, Ontario, Canada were deposited after the last Paleoproterozoic glacial event and an episode of intense chemical weathering. The Huronian Supergroup contains at the base ∼2.45 Ga volcanics and is intruded by the 2.22 Ga Nipissing diabase dykes and sills while the Chocolay Group is bracketed in age between ∼2.29 and 2.20 Ga. The Lomagundi (∼2.22-2.1 Ga) carbon isotope excursion started after the Paleoproterozoic glacial epoch and before a plume breakout event at 2.22 Ga. Therefore, the Chocolay and Upper Huronian carbonates were deposited either before or during the onset of the Lomagundi event. Notably, thin carbonates of the basal Gordon Lake Formation and thick carbonate succession of the Kona Dolomite in the northeastern exposures of the Chocolay Group record δ 13 C values as high as +9.5‰ versus V-PDB. Similar to other successions deposited during the Lomagundi event, both units contain pseudomorphs and molds after sulfates. This observation suggests that seawater sulfate contents rose dramatically in association with the onset of the Lomagundi event and the rise of atmospheric oxygen. Carbonates in the western and southern exposures of the Chocolay Group (Randville and Bad River dolomites, and Saunders Formation) previously assumed to be equivalent to the Gordon Lake Formation and Kona Dolomite have carbon isotope values close to 0‰. Based on basin analysis, we infer that these carbonate units were deposited during a negative carbon isotope excursion after the Lomagundi event started and are slightly younger than the Kona Dolomite and Gordon Lake Formation. This interpretation implies that the carbonate platform in the Great Lakes area transgressed to the west over shallow-marine and fluvial deposits. The negative carbon isotope excursion in the Lake Superior area might correspond to similar δ 13 C values of the Mooidraai Dolomite in the Griqualand West Basin, South Africa supporting correlation between Paleoproterozoic successions of North America and South Africa and the notion of three global glaciations in the Paleoproterozoic Era. Carbonates of the Mille Lacs Group (Trout Lake, Glen Township, and Denham formations) in Minnesota have δ 13 C values ranging from −1.2 to +2.5‰. Combined with geochronologic constraints, these data suggest that these units were deposited after the Lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized ∼2.0 Ga passive margin in the Lake Superior area.

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Chemostratigraphy of the Paleoproterozoic Duitschland Formation, South Africa: implications for coupled climate change and carbon cycling

American Journal of Science, 2001

The Paleoproterozoic Duitschland Formation lies stratigraphically beneath the Timeball Hill Forma... more The Paleoproterozoic Duitschland Formation lies stratigraphically beneath the Timeball Hill Formation, which contains the only unequivocal glacial unit of this era in the Transvaal Basin, South Africa. Lithologic evidence in Paleoproterozoic successions of North America, however, indicates the existence of three discrete and potentially global ice ages within this 300 my interval. Carbonates of the Duitschland Formation are significantly enriched in 13 C up to ؉10.1 permil in the upper part of the succession above a notable sequence boundary. In contrast, the lower part of this unit contains carbonates with consistently negative ␦ 13 C values. Trace and major element compositions of these carbonates as well as carbon-isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly. The stratigraphic constraints indicate that 13 C-enriched carbonates were deposited prior to Paleoproterozoic glaciation in southern Africa, similar to carbonates stratigraphically beneath Neoproterozoic glacial diamictites worldwide. Also mirroring the Neoproterozoic record are strongly negative ␦ 13 C values in cap carbonates atop glacial diamictites in Paleoproterozoic strata of Wyoming and Ontario. The litho-and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well-preserved carbonates of the lower Duitschland Formation may reflect a second Paleoproterozoic ice age in the Transvaal succession. This interpretation is further supported by recently discovered bullet-shaped clasts with striations in diamictite from the basal part of the succession. Thus, the emerging temporal pattern of carbon isotope variations and glaciation in the Paleoproterozoic has a close analogue to Neoproterozoic events, suggesting a coupling of climatic and biogeochemical changes at both ends of the eon. introduction The Paleoproterozoic Era (1.6-2.5 Ga) is a distinct interval in Earth history characterized by interrelated tectonic, evolutionary, climatic, and biogeochemical events resulting in broad changes of Earth's surface environments. The detailed relationship between key transitions is poorly understood, however, due to uncertainties in correlation and the age of sedimentary successions where critical events are preserved, as well as stratigraphic omissions and post-depositional insults that obscure the true nature of temporal trends. For example, in North America, three discrete levels of Paleoproterozoic glacial diamictites are recognized in the Huronian and Snowy Pass supergroups of Ontario and Wyoming (Young, 1973, 1988; Ojakangas, 1985), respectively; on other continents only a single unequivocal diamictite of glacial origin has thus far been recognized (Ojakangas, 1985). In North America, these ice ages are radiometrically constrained to have occurred over a 230 my interval between 2.22 and 2.45 Ga (fig. 1; Krogh, Davis, and Corfu, 1984; Corfu and Andrews, 1986).

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Echoes from the Past: A Healthy Baltic Sea Requires More Effort

AMBIO, 2014

Integrated sediment multiproxy studies and modeling were used to reconstruct past changes in the ... more Integrated sediment multiproxy studies and modeling were used to reconstruct past changes in the Baltic Sea ecosystem. Results of natural changes over the past 6000 years in the Baltic Sea ecosystem suggest that forecasted climate warming might enhance environmental problems of the Baltic Sea. Integrated modeling and sediment proxy studies reveal increased sea surface temperatures and expanded seafloor anoxia (in deep basins) during earlier natural warm climate phases, such as the Medieval Climate Anomaly. Under future IPCC scenarios of global warming, there is likely no improvement of bottom water conditions in the Baltic Sea. Thus, the measures already designed to produce a healthier Baltic Sea are insufficient in the long term. The interactions between climate change and anthropogenic impacts on the Baltic Sea should be considered in management, implementation of policy strategies in the Baltic Sea environmental issues, and adaptation to future climate change.

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Constraints for the latest glacial advance on Wrangel Island, Arctic Ocean, from rock surface exposure dating

Global and Planetary Change, 2001

During an expedition to Wrangel Island in 1997, three samples of quartz veins were collected for ... more During an expedition to Wrangel Island in 1997, three samples of quartz veins were collected for surface exposure dating. Two were bedrock surface samples from different parts of the island, one was from a local boulder. The samples were collected where former coverage by valley glaciers can be excluded. Especially, the bedrock surface samples were expected to provide new constraints for the chronology of glaciations on the island and on the arctic continental shelf in the Beringian region. Minimum exposure age estimates for these bedrock samples, calculated on the basis of 10 Be concentrations, are 64,600 " 6400 and 26,400 " 2100 years. These estimates suggest that no major glaciations have affected Wrangel Island or the adjacent shelf area after 64,600 " 6400 years. Specifically, these dates seem to rule out any major glaciation Ž. during the Last Glacial Maximum LGM. The estimates should however be regarded as preliminary, as only two bedrock surface samples were analysed, but are compatible with the absence of glacial morphology and sediments, and with radiocarbon dates from mammoth remains.

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A short-term, post-Lomagundi positive C isotope excursion at <i>c</i>. 2.03 Ga recorded by the Wooly Dolomite, Western Australia

Kumpaa on enemmän, vettä vai maata? Paljonko ero on kiloissa? (Lasten tiedekysymykset)

Geotieteellisiä tutkimusmatkoja sadan vuoden ajalta

Helsingin yliopistomuseossa Arppeanumissa avattiin 4.4.2008 nayttely suomalaisista geotieteellisi... more Helsingin yliopistomuseossa Arppeanumissa avattiin 4.4.2008 nayttely suomalaisista geotieteellisista tutkimusmatkoista sadan vuoden ajalta. Kaikkiaan nayttelyssa esitellaan yksitoista geotieteellista tutkimusmatkaa lahtien maantieteilija Johannes Gabriel Granon matkoista Mongoliaan 1900-luvun alkuvuosina ja paatyen suomalaisia komponentteja sisaltaneen Huygens-luotaimen laskeutumiseen Saturnuksen Titan-kuuhun vuonna 2005. Nayttely kuuluu osana Suomalaisen Tiedeakatemian 100-vuotisjuhlien tapahtumiin.

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