Biogenic Silica Content Research Articles

Impact of Holocene climate change on silicon cycling in Lake 850, Northern Sweden

Diatom-rich sediment in a small subarctic lake (Lake 850) was investigated in a 9400 cal. yr BP sediment record in order to explore the impact of Holocene climate evolution on silicon cycling. Diatom stable silicon isotopes ([Formula: see text]) and biogenic silica (BSi) indicate that high BSi concentrations in sediment throughout the Holocene are associated with a lighter Si isotope source of dissolved silica (DSi), such as groundwater or freshly weathered primary minerals. Furthermore, higher BSi concentrations were favoured during the mid-Holocene by low detrital inputs and possibly a longer ice-free period allowing for more diatom production to occur. The diatom [Formula: see text] signature shows a link to changes in regional climate and is influenced by length of diatom growth period and hydrological fluctuations. Lighter Si isotopic values occur during the mid-Holocene, when climate is inferred to be more continental and drier, with pronounced seasonality. In contrast, a heavier Si isotopic signature is observed in the early and late Holocene, when oceanic influences are thought to be stronger and the climate wetter. The [Formula: see text] values have generally lighter signatures as compared with other studies, which supports a light DSi source.

Biogenic matter content in marine sediments in the vicinity of the Antarctic Peninsula: Recent sedimentary conditions under a diverse environment of production, transport, selective preservation and accumulation

Burial fluxes of organic carbon and biogenic silica were determined in 17 continental shelf sediment cores collected from the northern Weddell Sea, the Bransfield Strait, and the southern Drake Passage. Coring sites included open-shelf stations as well as slope and glacial trough environments, with water depths varying from 220 to 760 m. Apparent 210Pb accumulation rates from these cores ranged from 0.04 g m−2y−1 to 0.21 g m−2y−1 (1 to 3 mm y−1), with organic carbon burial rates ranging from 3 to 15 g OC m−2y−1 and biogenic silica accumulation rates ranging from 15 to 126 g SiO2 m−2y−1. OC contents below the surface mixed layer ranged from 0.26 to 1.51 wt. % (avg. 0.64 %). Biogenic silica contents at depth ranged from 2.3 to 11.2 wt. % (avg. 7.5%), with an average bSi/OC ratio (wt. %/wt. %) at depth of 12. Annual OC primary production rates and biogenic silica production rates in the euphotic zone were estimated from satellite chlorophyll-a data in the literature and from a seasonal model for biogenic particle export from surface waters. Based on these biogeochemical data, preservation efficiencies (i.e., mass burial rate/water column production rate) were calculated for organic carbon and biogenic silica. These preservation efficiency values ranged from 2 to 18% (avg. 9%) for OC and 8 to 106% (avg. 54%) for bSi. These relatively high preservation efficiencies resulted from extensive lateral sediment focusing (210Pb Psi (Ψ) values [burial flux/water column production rate] ranging from 2 to 33; avg. of 16), cold bottom water temperatures (2 to −2°C), and relatively high biogenic Si and OC production rates in the euphotic zone. The enhanced preservation efficiency for bSi relative to OC (i.e., 54% vs. 9%) in these Antarctic settings is consistent with the change in the phytoplankton bSi/OC (wt. %/wt. %) value of 2 for this area to the burial bSi/OC value of 12. Excess 210Pb activities in surface sediments varied from 4 to 47 dpm g−1. The surface mixed layer in the seabed varied in thickness from 0 to 4 cm. The penetration of excess 210Pb into these Antarctic Peninsula sediments varied from 6 to 28 cm (avg. 18 cm). The inventory of excess 210Pb in the seabed varied from 13 to 230 dpm cm−2 (avg. 110 dpm cm−2). Although 210Pb was the only radionuclide measured in this study, “apparent” 210Pb sediment accumulation rate (SAR) values from these 17 cores (assuming that deep bioturbation is negligible) are believed to be accurate SAR values because of good agreement between 210Pb and 14C chronologies from nearby cores reported in the literature.

Modern silicon dynamics of a small high-latitude subarctic lake

Abstract. High biogenic silica (BSi) concentrations occur sporadically in lake sediments throughout the world; however, the processes leading to high BSi concentrations vary. We explored the factors responsible for the high BSi concentration in sediments of a small, high-latitude subarctic lake (Lake 850). The Si budget of this lake had not been fully characterized before to establish the drivers of BSi accumulation in this environment. To do this, we combined measurements of variations in stream discharge, dissolved silica (DSi) concentrations, and stable Si isotopes in both lake and stream water with measurements of BSi content in lake sediments. Water, radon, and Si mass balances revealed the importance of groundwater discharge as a main source of DSi to the lake, with groundwater-derived DSi inputs 3 times higher than those from ephemeral stream inlets. After including all external DSi sources (i.e., inlets and groundwater discharge) and estimating the total BSi accumulation in the sediment, we show that diatom production consumes up to 79 % of total DSi input. Additionally, low sediment accumulation rates were observed based on the dated gravity core. Our findings thus demonstrate that groundwater discharge and low mass accumulation rate can account for the high BSi accumulation during the last 150 cal yr BP. Globally, lakes have been estimated to retain one-fifth of the annual DSi terrestrial weathering flux that would otherwise be delivered to the ocean. Well-constrained lake mass balances, such as presented here, bring clarity to those estimates of the terrestrial Si cycle sinks.

Response of Geochemical Characteristics on the Environmental Changes in the Holocene: Data on Bottom Sediments of Lake Maloe, Iturup Island

Major and trace element distribution in the bottom sediments of Lake Maloe located on the Kuybyshev Isthmus, Iturup Island, was analyzed using the method of principal components. It was established that the geochemical characteristics of Lake Maloe sediments reflect a change from marine and lagoonal to fresh-water sedimentation environments. According to geochemical data, the erosion and weathering products of proximal rocks supplied detritus in the sedimentary basin. Marine facies (up to 6400 yr ago) are characterized by the increased contents of carbonates, the high salinity index Sr/Ba, and the low contents of nickel, biogenic silica, and organic matter. The element composition is clearly correlated with grain-size composition. The CIA index and Rb/Sr ratio indicate that the coarse-grained varieties, regardless of their genesis, are less chemically altered. They are enriched in detrital components, which comprise all elements, except for SiO2 and phosphorus. Marine regression at ca. 6400 yr ago was followed by lacustrine environment, which provided the predominant accumulation of biogenic silica and organic. The aeolian activity related to climatic cooling and marine regressions led to the formation of coarse-grained interlayers in silty sequence, which are clearly expressed by geochemical characteristics. The fresh-water facies show a distinct grain-size rhythmicity traced by Al2O3/(CaO + Na2O) ratio.

The impact of volcanic activity on the deposition of organic-rich shales: Evidence from carbon isotope and geochemical compositions

The organic-rich black shales from the Late Ordovician and early Silurian were identified as essential source rocks for shale gas exploration and development in South China. To investigate the influence of volcanic activity on the deposition of organic-rich shales in the Wufeng and Longmaxi formations, this study presents the carbon isotope and geochemical compositions of the Upper Ordovician-lower Silurian succession of the Yangtze Platform. The organic-rich Wufeng and lower Longmaxi shales are characterized by isotopically negative carbon shifts; however, obviously high δ13Corg values are shown in the organic-poor shales of the upper Longmaxi Formation. The negative δ13Corg excursion is attributed to 12C-depleted CO2 sourced from volcanic activity. Furthermore, the positive δ13Corg excursion in the upper Longmaxi Formation occurs because a large amount of 12C was removed from the ocean-atmosphere system through continuous burial of organic carbon. The biogenic silica (Sibio) contents and (Cu + Ni)/Al ratios in the lower Longmaxi Formation display higher values than those in the upper Longmaxi Formation, suggesting that the primary productivity gradually decreased from the Wufeng Formation to the Longmaxi Formation. This study proposes that high primary productivity was related to accelerated chemical weathering and release of micro-nutrients. The former emphasizes that high pCO2 and globally high temperatures can enhance chemical weathering, so that more nutrients are transported from continents to oceans (Yan et al., 2010, 2019). The latter highlights that volcanic material input can act as fertilizer by releasing many nutrients. Moreover, the values of UEF and MoEF are higher in the Wufeng and lower Longmaxi formations than in the upper Longmaxi Formation, suggesting that the oxygen concentration increased upward through the section. Given the positive correlations between indicators of productivity and redox conditions, the constantly high primary productivity would consume much O2, resulting in anoxic conditions in the Wufeng and lower Longmaxi formations. Overall, this study considers that volcanic activity-driven primary productivity is the first-order control for organic matter (OM) enrichment.

A 1 Million Year Record of Biogenic Silica in the Indian Ocean Sector of the Southern Ocean: Regional Versus Global Forcing of Primary Productivity

AbstractA new orbital‐scale record of bulk sediment biogenic silica (opal) content from Ocean Drilling Program (ODP) Site 745B spans the past 630 kyr (Marine Isotope Stages (MIS) 1–16). Together with the published portion of the record (MIS 16–31, Billups et al., 2018), we obtain a 1 million year orbital‐scale record of paleoproductivity in the Antarctic Zone of the Indian Ocean sector of the Southern Ocean. A coherent age model is based on tuning variations in the opal content to the benthic foraminiferal δ18O stack of Lisiecki and Raymo (2005). Consistent with other sites from the Antarctic Zone of the Southern Ocean, we align opal maxima with interglacial and opal minima with glacial intervals. Opal variations are significant at all three primary periodicities (100, 40, and 23 kyr), coherent (>95%), and in‐phase with the tuning target on the scale of individual cycles as well as their amplitude modulation. This supports the assumption of global ice volume‐related changes in Southern Ocean paleoproductivity in the derivation of the age model. Between MIS 11 and 4, however, the opal record lacks minima corresponding to the glacial extremes of MIS 10, 8, and 6. During this interval of time, opal fluctuates primarily with a 23 kyr precession signal. We suggest that the productivity response to precession reflects an ice‐free sea surface that remains sensitive to wind‐driven upwelling of nutrients. Results from Site 745B illustrate the potential importance of regional climate forcing factors on longer time scales and their interplay with global climate background conditions.

Late Holocene Peatland Evolution in Terelj and Tuul Rivers Drainage Basins in the Khentii Mountain Range of Northeastern Mongolia

This study reviews the late Holocene peatlands in Terelj River and Tuul River drainage basins in the Khentii (Khentii has been misspelled as Khentey (or Hentey) and Khentei (or Hentei) in many publications. The Khentii is the right English translation from Mongolian Xэнmuй) Mountain Range of northeastern Mongolia. The peatlands were examined through their physical and chemical properties, diatom assemblages, and radiocarbon dating. In the Terelj River basin, the high contents of organic matter and biogenic silica and the dominant benthic diatom assemblages such as Eunotia praerupta, Pinnularia borealis, and Navicula mutica in the peat deposits indicate the warm and humid climates in the late Holocene. The high accretion rate of 0.97 mm/yr in the peatland records the intensive erosion in the surrounding landscape and deposition in the peatland due to increased precipitation and runoff in the humid climate since 0.5 cal. ka BP. In the Tuul River basin, the high content of mineral fractions and diatom assemblages dominated by benthic species Cymbella proxima, Encyonema silesiacum, and planktonic species Cyclotella ocellata in the peat deposits show a transition from humid to arid climates at 0.9 cal. ka BP. The accretion rate of 0.56 mm/yr in the peatland on the paleo-floodplain indicates strengthened erosion in the peatland over the past ~1000 years. This study in the southern Khentii Mountain Range provides new descriptive insights to extend the underestimated Mongolia’s peat studies, and it would be a useful proof-of-concept study for future detailed paleo-environmental analyses.

Mechanism and implications of upwelling from the Late Ordovician to early Silurian in the Yangtze region, South China

Oceanic upwelling is an important process in ocean cycling and has implications for climate change and ocean productivity. During the Late Ordovician to early Silurian (O/S) transition, many dramatic geologic events occurred, including the Hirnantian Glaciation, enhanced organic matter accumulation, and upwelling events. Upwelling is thought to be closely linked to contemporary geologic events; however, the internal relationships and mechanisms involved remain poorly understood due to a reliance on qualitative evidence and low-resolution data. To better determine the presence and development of upwelling, as well as its underlying mechanisms and potential implications for environmental changes during the O/S transition, we report new evidence and insights based on data for South China. Detailed petrographic and geochemical analyses of a well-sequenced drill core from the Lower Yangtze region are presented and compared with other extensively examined sections from the Upper Yangtze region. We show that the excessive biogenic silica content associated with certain geochemical fingerprints (i.e., Cd/Mo, Co × Mn, and CoEF × MnEF) offer compelling evidence for widespread upwelling in the Yangtze region during the O/S transition. This upwelling was strong during the late Katian to Hirnantian, significantly weaker during the early Rhuddanian, and was generally more intense in the relatively deep waters of the Yangtze region. The glacially-influenced temperature difference between the poles and equator, as well as longshore trade winds, may have been the drivers of this widespread upwelling. The upwelling event may have increased surface productivity, driving the accumulation of organic matter in the Wufeng Formation in South China. Moreover, the upwelling event can be regarded as a broad indicator of Late Ordovician glaciation and may be the geochemical basis for the Boda warm Event that occurred prior to Hirnantian cooling.

A record of diatom community response to catchment land-use change in Moreton Bay, Australia

Embayments and the biota they support are highly susceptible to disturbance within adjacent catchments. Examining the timing and magnitude of impacts arising from human-induced disturbance in these systems is often limited due to the absence of long-term monitoring. Moreton Bay in south-eastern Queensland is a shallow embayment that receives inflows from a 21220-km2 catchment. In this study, diatom abundances, pigment (chlorophyll-a, phaeopigments) and biogenic silica concentrations were evaluated in a composite sediment from central Moreton Bay to explore temporal trends in the photosynthetic community. The record extends from 1200 to 2011, incorporating the period of European settlement (c. 1840s) and rapid population growth and urbanisation of the catchment. The record shows that in central Moreton Bay bloom-forming marine diatoms (Thalassiosira, Thalassiothrix, Thalassionema) have increased in relative abundance since the mid-20th century, whereas the dominant benthic diatom (Paralia fenestrata) has declined. This transition most likely occurred in response to the compounding effects of increased delivery of nitrogen, fine sediments and pollutants to Moreton Bay as a consequence of changes in land use. The inferred historical decline in relative contributions of benthic microalgae to total primary production in central Moreton Bay has likely had wide-ranging ecological effects.

Sedimentary Nutrient Supply in Productive Hot Spots off the West Antarctic Peninsula Revealed by Silicon Isotopes

In this study we evaluate the benthic fluxes of silicic acid along the West Antarctic Peninsula (WAP). Silicic acid (DSi) is one of the macronutrients essential in fuelling biological hot spots of diatom‐dominated primary production along the WAP. Here we measure the concentration and stable silicon isotopic composition of DSi in porewater profiles, biogenic silica content (BSi), and diatom abundance from sediment cores collected along the WAP. We couple these measurements with reaction‐transport modeling, to assess the DSi flux and the processes that release this key nutrient from the sediment into the overlying waters. Our results show that the benthic DSi flux is dominated by the diffusive flux, which is estimated to be equivalent to 26.7 ± 2.7 Gmol yr−1 for the WAP continental shelf. The DSi isotope profiles reveal the important impact of sedimentary processes on porewater DSi and suggest that biogenic silica dissolution is the main source of DSi in porewaters and consequently of the benthic fluxes. Our integrated data‐model assessment highlights the impact of surface productivity on sedimentary processes and the dynamic environment of core‐top sediments where dissolution and reverse weathering reactions control DSi exchanges.

Effect of diatom abundance and biogenic silica availability on the population growth of tintinnid ciliates at Suruga Bay

The effects of diatom abundance and biogenic silica (BSi) deposition on the population growth of tintinnid ciliates were investigated using incubations of sub-surface waters (10 m depth) of Suruga Bay, Japan in April and November 2018. Seawater for incubation was pre-filtered using a 200-µm mesh to remove mesozooplankton grazers. Initial concentrations of dissolved inorganic nitrogen were similar in both seasons, but silicic acid and phosphate were higher in April than in November. This allowed us to observe the growth of planktonic communities under varied silicic acid availability and explore the effects of BSi accumulation and food availability on the growth of tintinnids. Phytoplankton blooms dominated by diatoms formed in both, April and November incubations, but their biomass was higher in April than in November, demonstrating the effects of nutrients, particularly silicic acid, on diatom growth. The BSi concentration increased in both incubations but was higher in April than in November, confirming the effects of diatom abundance and size on BSi deposition. Tintinnid biomass also increased in both incubations but was higher in April than in November, showing that the biomass of tintinnid was affected by the BSi concentration. Species composition of diatoms affected the species composition of tintinnids by providing selected material for the construction of their lorica. Moreover, the results showed that the food supply mainly provided by pico-nanoflagellates via microbial loop and detritus may limit tintinnids’ population growth in natural planktonic ecosystems and this demonstrates tintinnids’ role as top predators in the microbial loop

Dynamics of biogenic silica accumulation and ecological characteristics in single-species communities and ecotones in Min River estuary, China

The role of silicon in plant resistance to biotic and abiotic stresses is clear; however, its role in interspecific interactions is not well understood. Biogenic silica (BSi) accumulation and ecological characteristics in single-species communities (Phragmites australis, Cyperus malaccensis, and Spartina alterniflora) and ecotones (P. australis–C. malaccensis and C. malaccensis–S. alterniflora) of Shanyutan marsh, China, were monitored from January to December in 2016. The BSi content of the three plant species decreased at the end of winter and beginning of spring, and continued to increase after March. In ecotones, the density of P. australis, the lengths of C. malaccensis and S. alterniflora, and the BSi content of C. malaccensis were greater than those in single-species communities. However, in single-species communities, the densities of C. malaccensis and S. alterniflora, the length of P. australis, the biomass and BSi stocks of the three species, and the BSi content of P. australis and S. alterniflora were greater than those in the ecotones. The three species may apply different strategies to compete for resources during interactive growth. Phragmites australis may improve its competitive ability by increasing vegetation density, aboveground biomass, and Si allocation to the leaves and withered body. Spartina alterniflora appears to enhance root biomass accumulation and the Si uptake and allocation capacity of roots. Cyperus malaccensis appears to allocate greater biomass and BSi to aboveground organs, as well as improve the absorption capacity of roots to resist competition pressure from P. australis. Cyperus malaccensis mixed with S. alterniflora increased its belowground biomass and BSi stocks. These results help clarify the mechanisms and processes of Si translocation during mixed plant growth, and increase our understanding of the strategies involved in plant competition.

The diatom response to Postglacial environments in the White Sea, the European Arctic

The White Sea is a small semi-enclosed Arctic sea influenced by Atlantic waters. It was completely covered by the Scandinavian Ice Sheet in the Late Pleistocene and plays a key role in the reconstruction of the degradation of the Scandinavian Ice Sheet and Atlantic Water advection into the Arctic Ocean. We examined the inflow of Atlantic water into the White Sea Basin as the leading feature of postglacial sedimentation processes. Palaeoreconstructions were conducted through detailed studies of diatom assemblages in two sediment cores from the central area (PS-6042) and outer part (PS-6050) of Dvina Bay. Stratigraphic division of the core sediments was performed on the basis of palynostratigraphy and accelerator mass spectrometry 14C dates. The composition of diatom assemblages in postglacial sediments from Dvina Bay indicate a change in the environment from a freshwater sea bay to fully marine conditions over the past 12 cal kyr. Based on the obtained results, we established that the Atlantic water started to flow into the central Dvina Bay in the first half of the Preboreal (11.7–11.3 cal ka BP), and the greatest increase in water surface temperatures in central Dvina Bay occurred during the period from the end of the Atlantic to the first half of the Subboreal. We confirmed these conclusions through comparison with previously obtained results of aquatic palynomorphs, pollen and spores, grain size analysis, and measurements of total organic carbon and biogenic silica contents, as well as through correlation with published palaeoreconstructions of the White Sea and the Arctic Ocean.

Biogenic silica concentration as a marine primary productivity proxy in the Holsteinsborg Dyb, West Greenland, during the last millennium

We analyzed the biogenic silica (BSi) content and produced a diatom-based summer sea-surface temperature (SST) reconstruction for sediment core GC4 from the Holsteinsborg Dyb, West Greenland. Our aim was to reconstruct marine productivity and climatic fluctuations during the last millennium. Increased BSi content and diatom abundance suggest relatively high marine productively during the interval of AD 1000–1400, corresponding in time to the Medieval Warm Period (MWP). The summer SST reconstruction indicates relatively warm conditions during AD 900–1100, followed by cooling after AD 1100. An extended cooling period during AD 1400–1900 is characterized by prolonged low in reconstructed SST and high sea-ice concentration. The BSi values fluctuated during this period, suggesting varying marine productivity during the Little Ice Age (LIA). There is no significant correlation between the BSi content and SST during the last millennium, suggesting that the summer SST has little influence on marine productively in the Holsteinsborg Dyb. A good correspondence between the BSi content and the element Ti counts in core GC4 suggests that silicate-rich meltwater from the Greenland ice sheet was likely responsible for changes in marine productively in the Holsteinsborg Dyb.

Seasonal dynamics and export of biogenic silica in the upper water column of a large marginal sea, the northern South China Sea

Biogenic silica (BSi), primarily derived from marine diatoms, is a major component of the oceanic silicon cycle. It links with the global carbon cycle through the silicate pump via photosynthesis and the so-called “ballast” effect in many areas of the world’s oceans, particularly in ocean margins. Here we examined the distribution and export of BSi based on a large data set collected from the upper water column of the northern South China Sea (NSCS), the largest marginal sea of the Pacific Ocean. Generally, BSi concentrations decreased from the shelf to the basin in all seasons. The highest value was observed in nearshore surface waters in summer, associated with a diatom bloom in the far-reaches of river plumes. Seasonally, BSi concentrations were overall higher in autumn and winter than in spring and summer, due to elevated diatom productivity supported by an increased vertical supply of nutrients in dry seasons. Patterns of BSi spatial and seasonal variability were well represented by those of the pigment fucoxanthin, demonstrating the dominant role of diatoms in controlling BSi dynamics in the upper NSCS. Mainly regulated by BSi standing stock, BSi export flux from the euphotic zone, determined using thorium-234/uranium-238 disequilibrium, was on average higher on the shelf than in the basin, and in the dry than wet season. Furthermore, the high BSi export helped to promote the export of particulate organic carbon (POC) on the shelf, while the POC standing stock was comparable between different regions of the NSCS. Thus, the silicate pump, driven by diatoms in the upper water column, functioned more efficiently under eutrophic conditions. A comparison with prior measurements of particle sinking fluxes using sediment traps showed that the correlation of BSi and POC increased and became more significant with increasing water depth. This provides direct evidence of the pronounced ballast effect of BSi in transferring POC into deep waters of the NSCS basin, even though the contribution of diatoms to POC accumulation in the euphotic zone was minor in this oligotrophic environment.

Silica diagenesis in the Lower Paleozoic Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for reservoir properties and paleoproductivity

Silica diagenesis is one of the most important aspects of black shale diagenesis and critically controls the reservoirs properties of black shales. Three stratigraphic sections of the Wufeng and Longmaxi Formations in the Sichuan Basin along a proximal to distal transect were investigated to study quartz types and their silica sources, as well as their influences on reservoirs properties including rock mechanical properties and porosity preservation. Six types of quartz including detrital quartz, recrystallized radiolaria, siliceous fossil fragments, microcrystalline quartz, quartz overgrowth, and hydrothermal quartz veins were identified based on their occurrences and morphologies under the petrographic microscope and scanning electron microscope and cathodoluminescence color and intensity. Original biogenic silica now exists as recrystallized radiolaria and microcrystalline silica in the mudstone matrix and follows systematic distribution patterns both laterally and stratigraphically. The biological origin of this silica component is supported by the presence of recrystallized radiolaria with preserved spines, preservation of “ghosts” of the central capsule, serrated edges reflecting the spherical lattices of the outer shell, and in addition preserved sponge spicules. A further source of silica for microcrystalline quartz formation was silica released in the course of clay mineral (smectite to illite) transformation.The different types of quartz make different contributions to the reservoir properties of shales, with microcrystalline quartz having the most important impact. Microcrystalline quartz enhances the rock mechanical properties of shales and protects initial porosity from mechanical compaction through a rigid framework of interconnected diagenetic silica grains. Pores between microcrystalline quartz grains are shielded from complete mechanical compaction and provide space for accumulation of bitumen and oil later in burial history. Subsequently, these pore-filling organic matter develop secondary organic nanopores at higher thermal maturity.Although biogenic silica content has been used as a paleoproductivity proxy, the method used to calculate it is based on questionable assumptions, and model parameters need to be fine-tuned for a given shale succession on the basis of petrographic observations. In addition, the commonly observed positive correlation between total organic carbon (TOC) and biogenic silica is invalid for very low sedimentation rates and extremely high concentrations of biogenic silica. From low to extremely high biogenic silica content in organic-rich sediments, a parabolic relationship between TOC and biogenic silica should be common.

The influence of rigid matrix minerals on organic porosity and pore size in shale reservoirs: Upper Devonian Duvernay Formation, Alberta, Canada

Organic porosity is an important component of the pore system in many shale hydrocarbon reservoirs. While much research has been directed towards understanding the development of organic porosity as a function of thermal maturity, relationships between mineralogy and organic porosity have been less extensively investigated. This study illustrates the role of rigid matrix minerals, particularly biogenically-sourced microcrystalline quartz, on organic porosity and pore size evolution.The Upper Devonian Duvernay Formation of western Canada is a siliceous-calcareous self-sourced reservoir with 3.2-3.79 wt. % average total organic carbon (TOC) content in a study area occurring in the volatile oil to dry gas thermal maturity zones. Biogenic silica is widely distributed throughout the matrix as microcrystalline quartz and locally comprises >50 wt. % of the rock. Porosity and pore size data gathered from BIB- and FIB-SEM, helium porosimetry, H1 NMR T2, and MICP indicate that organic porosity and pore size is enhanced when in association with biogenic silica. Biogenic silica forms a rigid siliceous matrix framework that limits compaction of pores and ductile organic matter and clay minerals. Where biogenic silica concentration is low, clay mineral (mica+illite) interparticle micro- and fine mesopores dominate. In these samples detrital quartz and biogenic silica have limited influence on pore size other than to preserve the small fraction of organic macropores. The occurrence of biogenic- and clay-dominated pore systems varies systematically with stratigraphic and lateral variations in lithofacies, and sharp contrasts are observed across sequence boundaries. The observation that mineralogy strongly influences organic pore preservation confirms that studies examining the relationship between thermal maturity and organic porosity must account for mineralogical variations.

Holocene temperature and landscape history of southwest Greenland inferred from isotope and geochemical lake sediment proxies

Terrestrial temperature reconstructions from the ice-free margins of Greenland are critical for constraining the sensitivity of the Greenland ice sheet to past climate change. Numerous investigations have clarified the glacial history of southwest Greenland in recent decades; however, very few local quantitative temperature reconstructions exist as context for changes in the cryosphere. We examine sedimentary records from two lakes south of Nuuk, Greenland (informally named lakes T1 and T2), and investigate millennial scale Holocene climate change using a multi-proxy approach. Changes in X-Ray fluorescence spectroscopy (XRF), δ15N, δ13C and biogenic silica concentrations of bulk lake sediments mark the transition from a marine environment to isolated lakes as relative sea level fell in the early Holocene. δ18O values of subfossil insects (chironomids) from lakes T1 and T2 decreased by 2–3‰ from c. 8.8 ka BP (minimum limiting age for the onset of lacustrine sediment deposition in lake T2; ka BP = thousands of years before 1950) to 1 ka BP. Existing isotope-independent temperature reconstructions from the west coast of Greenland suggest that these changes in δ18O values accompanied gradual cooling of 2–4 °C, concomitant with decreasing summer insolation. There is widespread regional evidence that this cooling resulted in progressive regrowth of local glaciers and of the ice sheet. The regional consensus on the magnitude of this temperature change indicates that the local, modern modeled temperature - precipitation δ18O relationship cannot be used to quantify millennial-scale temperature changes at these sites through the Holocene. This reinforces that studies utilizing δ18O-based proxies to infer temperature change should carefully assess local isotope-temperature relationships over space and time.

Integrating palaeoclimate time series with rich metadata for uncertainty modelling: strategy and documentation of the PalMod 130k marine palaeoclimate data synthesis

Abstract. Palaeoclimate data hold the unique promise of providing a long-term perspective on climate change and as such can serve as an important benchmark for climate models. However, palaeoclimate data have generally been archived with insufficient standardisation and metadata to allow for transparent and consistent uncertainty assessment in an automated way. Thanks to improved computation capacity, transient palaeoclimate simulations are now possible, calling for data products containing multi-parameter time series rather than information on a single parameter for a single time slice. Efforts are underway to simulate a complete glacial–interglacial cycle using general circulation models (https://www.palmod.de/, last access: 6 May 2020), and to confront these simulations with palaeoclimate data, we have compiled a multi-parameter marine palaeoclimate data synthesis that contains time series spanning 0 to 130 000 years ago. We present the first version of the data product that focuses exclusively on time series for which a robust chronology based on benthic foraminifera δ18O and radiocarbon dating is available. The product contains 896 time series of eight palaeoclimate parameters from 143 individual sites, each associated with rich metadata, age–depth model ensembles, and information to refine and update the chronologies. This version contains 205 time series of benthic foraminifera δ18O; 169 of benthic foraminifera δ13C; 131 of seawater temperature; 174 and 119 of planktonic foraminifera δ18O and δ13C; and 44, 38 and 16 of carbonate, organic carbon and biogenic silica content, respectively. The data product is available in three formats (R, LiPD and netCDF) facilitating use across different software and operating systems and can be downloaded at https://doi.org/10.1594/PANGAEA.908831 (Jonkers et al., 2019). This data descriptor presents our data synthesis strategy and describes the contents and format of the data product in detail. It ends with a set of recommendations for data archiving.

Diagenetic formation of bedded chert: Implications from a rock magnetic study of siliceous precursor sediments

Bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz-rich chert beds and clay-rich interbeds. The origin of the lithologic variations is not well understood. A rock magnetic study of a sequence of alternating bands of radiolarian-rich and clay-rich bands recovered by ODP leg 129, which is considered to be the best-known analog for the bedded chert precursor sediments, shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types, with only a minor (1.5 times on average) increase in bulk concentration in clay-rich bands. Comparison with magnetic properties of the diagenetically mature chert-shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification. A simple silica transfer model, with dissolution of silica in proto-shale donor beds and deposition in proto-chert receiver beds, indicate that the known features of the bedded chert sequences, such as the difference in thickness, cosmic dust content between chert and shale, can be reproduced in case of donor beds having initially lower clay (higher biogenic silica) content compared to receiver beds. While contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays, an association of proto-shale beds with high productivity episodes is supported by an independent paleoproductivity proxy – their higher Al/Ti values, and is in full agreement with the known silica diagenesis trends, particularly, with the retarding influence of detrital minerals on silica transformations. The impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession. Silica dissolution starts in clay-poor intervals. When diffusing silica encounters clay-rich beds, adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz. Early deposition of quartz cement prevents significant compaction of chert layers, while the loss of silica in donor beds leads to their compaction and concomitant increase in non-reactive clay and cosmic dust particles. Severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies.