Mapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification

Scientific Reports volume 13, Article number: 1516 (2023) Cite this article 680 Accesses 1 Altmetric Metrics details Silica cycling in the world’s oceans is not straightforward to evaluate on a

Scientific Reports volume 13, Article number: 1516 (2023) Cite this article

680 Accesses

1 Altmetric

Metrics details

Silica cycling in the world’s oceans is not straightforward to evaluate on a geological time scale. With the rise of radiolarians and sponges from the early Cambrian onward, silica can have two depositional origins, continental weathering, and biogenic silica. It is critical to have a reliable method of differentiating amorphous silica and crystalline silica to truly understand biogeochemical and inorganic silica cycling. In this study, opal-A is mapped across the Western Canada Sedimentary Basin in the Late Devonian Duvernay Formation shales using longwave hyperspectral imaging alongside geochemical proxies that differentiate between crystalline and amorphous SiO2, during the expansion of the world’s early forests. Signaled by several carbon isotope excursions in the Frasnian, the punctata Event corresponds to the expansion of forests when vascular land plants develop seeds and deeper root networks, likely resulting in increased pedogenesis. Nutrients from thicker soil horizons entering the marine realm are linked to higher levels of primary productivity in oceans and subsequent oxygen starvation in deeper waters at this time. The results of this study reveal, for the first time, the spatial distribution of amorphous SiO2 across a sedimentary basin during this major shift in the terrestrial realm when forests expand and develop deeper root networks.

A major shift in climate and oxygen levels in Earth’s atmosphere beginning near the Emsian-Eifelian boundary (~ 395 Ma)1 and continued into the Early Frasnian when forests were expanding2,3,4. The world’s first forests were identified in the late Emsian in Spitzbergen and in Givetian strata in Gilboa, New York, USA5,6, however, Capel et al.3 identifies several major origination-extinction pulses during the Silurian-Devonian that eventually resulted in a transition to a forested terrestrial landscape during the Middle Devonian. By the end of the Givetian, root networks had deepened and by the Frasnian, aneurophyte and archaeopterid progymnosperm forests were common, resulting in thicker soil horizons starting to form; thereby increasing terrestrially-derived nutrient delivery to the marine environment2,4,7. Previous studies of these shifts in biodiversity predicted that enhanced nutrient delivery may have caused increases in productivity, oxygen stratification, deposition of organic rich black shales, and eutrophication in Frasnian epicontinental seas2,4,8,9. Middle to Late Devonian lacustrine sediments from Greenland and northern Scotland reveal a net loss in phosphorous (P), an essential biolimiting nutrient that is expected to decrease in a terrestrial environment that is undergoing plant colonization where P is liberated from minerals indirectly through the acidification of root pore spaces produced by the degradation of organic matter and release of organic exudates from roots8,10,11. A significant and long-lasting δ13C shift in the punctata conodont zone, thought to be caused by the increased delivery of liberated nutrients (e.g. P) that would enhance productivity and burial of organic carbon in the Middle to Late Devonian, is referred to punctata Event (pE) and is recognized in basins worldwide12. Suspected productivity associated with the pE may also result in amplification of biologically sourced amorphous SiO2 in areas that experienced nutrient influx through the delivery of soils formed by deeper root networks2,8. Amorphous SiO2 has been consistently underestimated in ancient sedimentary sequences, which distorts our understanding of global biogeochemical silica cycling13,14,15,16. Silica in shales was commonly interpreted as terrigenous in origin; however, Schieber17 and Schieber et al.14 demonstrated that significant proportions of quartz silt in shales could be biogenically- or diagenetically-derived, especially after the early Cambrian when radiolarians and siliceous sponges started to proliferate16. The SiO2-rich Frasnian Duvernay Formation shales displays δ13C(org) excursions characteristic of the pE, which have also been documented in the Canadian Rocky Mountains18. These basinal deposits are therefore examined in this study to determine whether SiO2 in the Duvernay shales is of biological origin and whether increases in SiO2 deposition could be linked to the significant shift in the terrestrial realm when the world’s forests were expanding.

Differentiation of SiO2 polymorphs is possible but is challenging on a macroscale. Currently the methods used to differentiate amorphous (possibly biogenic) versus crystalline SiO2 are:

Identification of siliceous microfossils or SiO2-filled cysts17,19, textures recognized in petrographic work such as irregularly shaped grains with embayments and pointy projections17,20, pyrite inclusions17,19, and quartz grains with colloform or chalcedonic textures14,17;

Scanning electron microscope cathodoluminescence imaging or energy dispersive x-ray spectroscopy14,17,19,21,22;

Oxygen isotope values14,23;

Silica excess; defined by Rowe et al.24 as the absolute value of the difference between the measured silica content and the silica versus aluminium regression line, which represents silica in the aluminosilicate phase (i.e., clay minerals)20;

A negative correlation between silica and zirconium25;

A positive correlation between silica and TOC20;

X-ray diffraction, where peak height differences are used as a crystallinity index26

Alkaline digestion27.

Each of these methodologies requires spot analysis where a specific interval is targeted through sampling. To detect SiO2 on a larger scale, our study uses longwave infrared spectroscopy (LWIR) in the 8–12 µm wavelength range28. LWIR can differentiate between amorphous and crystalline SiO2 due to asymmetric stretching of Si–O–Si bonds in amorphous SiO229,30. Using this method of detection can enhance our understanding of biogeochemical SiO2 cycling through time, since it enables in situ detection on the macroscale, thereby allowing us to map amorphous SiO2 distributions in ancient sedimentary basins and track shifts in bioproductivity that may coincide with significant climatic shifts.

Three drill cores through the Duvernay shales from different locations across the Western Canada Sedimentary Basin in Alberta were sampled at regularly spaced intervals for whole-rock geochemistry and stable isotope analyses. Shales are less susceptible than carbonate rocks to diagenetic alteration that may affect δ13C, used to identify the pE. Each core is analysed for δ13C(org) to determine if the pE excursions are recorded in each of these locations. SiO2 provenance is investigated using oxide data and geochemical proxies to determine whether there is any “excess silica”24 present in the Duvernay and if so, what the source may be. Any SiO2 that is contributed from a biological source, signalling bioproductivity associated with the pE, would likely be an amorphous polymorph of SiO2. To detect and map amorphous SiO2, the cores are imaged using a longwave infrared (LWIR) drill core hyperspectral scanner. The aims of the study are to determine whether the pE is recorded at each location in the basin, to identify intervals of amorphous SiO2, and to determine the source of SiO2 using several geochemical proxies and hyperspectral imaging. Possible sources of amorphous SiO2 include hydrothermal fluids, SiO2 produced as a by-product of clay diagenesis, or biogenic SiO2. A biogenic source of SiO2 in the Duvernay, deposited during expansion of the world’s forests, would support current theories that the development of deeper Archaeopteris tree roots resulted in increased soil genesis and riverine nutrient delivery8, ultimately increasing bioproductivity and deposition of organic material in the oceans at this time2,4,12. This study, focused on differentiation and mapping of SiO2 polymorphs contributes to our understanding of silica in the world’s oceans through time, which is linked to global carbon, oxygen and climate cycles27. The novel technique used to differentiate detrital versus amorphous SiO2 may provide a critical advancement in our ability to quantify biogenic SiO2 in sediment, which is considered a high priority in developing a better understanding of biogeochemical silica cycling8,27.

The Duvernay Formation shales are organic-rich, calcareous to argillaceous/siliceous shales that were deposited during the Frasnian-age (382–372 Ma) Woodbend Group (Fig. 1), when the WCSB was a passive margin on the western passive margin of the North American craton31. The most up to date conodont stratigraphy from the WCSB places the lower portion of the Duvernay in the punctata conodont zone (Montagne Noire (MN) 5/6) and the upper Duvernay mostly in the hassi conodont zone (MN7-10)31. During this time, present-day Alberta was covered by a large interior seaway, with numerous carbonate buildups that grew in succession. The Leduc Formation formed on paleotopographic highs on the thickest portions of the underlying Swan Hills Formation platform in the west, and the Cooking Lake Formation platform in the east32,33. The first two stages of reef growth in the Leduc Formation are coeval with deposition of the Duvernay basinal mudstones and shales33. In the east, the Duvernay directly overlies the Cooking Lake Formation platform carbonates and in the west, the basinal equivalent Majeau Lake Formation. These western and eastern portions of the WCSB, known as the West Shale Basin and East Shale Basin respectively, are separated by the Rimbey-Meadowbrook reef trend (Fig. 1)34.

(A) Map of Alberta with extent of the subsurface Duvernay Formation. modified from the Geological Atlas of the Western Canada Sedimentary Basin28. (B) Simplified stratigraphy of the Duvernay Formation in Alberta, Canada and the location of three drill cores used in this study.

The Duvernay is highly heterogeneous both locally and across its 130,000 km2 depositional extent35. The Duvernay, or the Perdrix as it is referred to in the Rocky Mountain exposures, is comprised of ten lithofacies, defined by composition, grain size, and sedimentary structures in Knapp et al.36. Broadly, the Duvernay is composed of organic rich siliceous mudstones, carbonate, and clay-rich shales36. Based on informal litho- and chemostratigraphy and areas of production from the Duvernay, it is divided into three domains herein referred to as: Kaybob, Willesden Green (WG), and East Shale Basin (ESB) (Fig. 1). In the Kaybob area, the Duvernay has seven geochemically distinct units, and is characterized by the presence of a middle carbonate unit37,38. In our study, the Kaybob well is divided into the upper Duvernay, middle Duvernay (carbonate-rich), and the lower Duvernay. In the Willesden Green (WG) area, the Duvernay is comprised of thin, intercalated shale and fine-grained carbonate beds. In the East Shale Basin (ESB), the Duvernay is surrounded by the Leduc Formation reefs and as a result is relatively carbonate rich throughout, in comparison to the WG and the upper and lower Duvernay in the Kaybob well.

Three Duvernay Formation cores were sampled and imaged in this study; 100/04-19-064-22W5/00 (Kaybob), 100/09-25-039-06W5/00 (Willesden Green = WG), and 100/08-29-031-23W4/00 (East Shale Basin = ESB; Fig. 1).

A total of 303 samples were taken at 0.5–1 m intervals from three Duvernay drill cores. The samples were analysed for major elements using inductively coupled plasma optical emission spectrometry (ICP-OES). Samples from the Kaybob drill core were analysed for elemental chemistry using an iCAP 7000 Series ICP-OES (Thermo Fisher Scientific, Waltham, MA, USA) at Chemostrat Inc. laboratories in Houston, Texas, following the procedure outlined in Hildred et al.39. Samples from the Willesden Green and East Shale Basin cores were analysed using a Spectro ICP-OES (Perkin Elmer, Waltham, MA, USA) at Bureau Veritas Mineral Laboratories in Vancouver, British Columbia. Oxides analysed in the samples include: SiO2, TiO2, Al2O3, Fe2O3, MgO, MnO, CaO, Na2O, K2O, and P2O5. Total organic carbon (TOC) and δ13C(org) (reported relative to VPDB) were analysed at Chemostrat Inc. for the Kaybob well, and at the University of Saskatchewan Stable Isotope Laboratory for the WG and ESB. All of the samples analysed for δ13C(org) and TOC were acidified to remove carbonate material, homogenized, and the organic material oxidized to carbon dioxide, nitrogen bearing gases, and water, which are further separated for analysis. Isotopes were measured at Chemostrat Inc. using a Europa Scientific 2020 Isotope Ratio Mass Spectrometer and at the Saskatchewan Stable Isotope Laboratory using a Thermo Finnigan Flash 1112 EA coupled to a Thermo Finnigan Delta Plus XL through a Conflo III, where they were calibrated against international standards L-SVEC (δ13C(org) = − 46.6 VPDB) and IAEA-CH-6 (δ13C(org) = − 10.45 VPDB) with a lab reported precision of 0.12% (n = 18, 2σ). Fifty polished thin sections were prepared and examined from the Kaybob well. Each section was impregnated with blue epoxy to highlight pore space, and the sections were half stained with Alizarin red to recognize calcite and aragonite. The thin sections were examined using a Zeiss Axioscope A1 at MacEwan University in Edmonton, Alberta. Twenty-five samples from the Kaybob well were also analysed for mineral content using X-ray Diffraction (XRD) at the Chemostrat Inc. laboratories in Houston, Texas.

SiO2 and Al2O3 data were used to identify the presence of excess silica that is not considered to have been derived from continental crust, using Eq. (1)20.

A value of 3.527 was used for average shale or background40. The amount of excess silica (SiEX) is reported alongside δ13C(org) data. The most recently reported conodont biostratigraphy presented in Wong et al.31 was used to correlate δ13C(org) data collected in the Duvernay with previous studies of the punctata Event12. Data analysis was performed on oxides and TOC analysed in the three wells using multivariate statistics (Principal Component Analysis; PCA) in DataDesk® 6.3.1. to determine SiO2 provenance. PCA analyzes the total variance of the oxide and TOC dataset to determine the relationship of SiO2 to clay-associated oxides (e.g. Al2O3, TiO2, K2O, MgO, Fe2O3, Na2O), carbonates (e.g. CaO, MnO), or to variables that represent a biological influence (e.g. P2O5, TOC)41. In this study, eigen vectors e1 versus e2 and e1 versus e3 were plotted for each well to account for greater than 85% of variance in the dataset.

A potential hydrothermal source for amorphous SiO2 is evaluated using an Al–Fe-Mn ternary plot. Adachi et al.42 defines a zone that implies a hydrothermal influence near the Fe-apex (up to 30% Mn) and non-hydrothermal as being Al-rich, and Mn-poor. Samples from each well in this study are plotted on this ternary diagram.

Clay diagenesis (e.g. K-metasomatism) can produce amorphous SiO2 as a by-product43. To determine whether the clays in the Duvernay have undergone significant diagenetic alteration that may have introduced amorphous SiO2, oxide data is used to first determine the chemical index of alteration (CIA), which assesses the degree of weathering that the sediments have undergone from their source (Eq. 2)44,45.

CaO* should represent Ca only in the silicate portion44,45. Following McLennan44, CaO was corrected for using phosphate data where CaO* is equal to the moles of CaO minus moles of P2O5 × 10/3. This value is compared to the moles of Na2O, and if the corrected value of CaO* is less than moles of Na2O, than this CaO* value is used in Eq. 2, otherwise CaO* is equal to Na2O.

CIA values calculated for sediments that have experienced diagenesis through K-metasomatism need to be corrected using an A-CN-K (Al2O3–CaO* + Na2O–K2O) plot where molar values of Al2O3, (CaO* + Na2O), and K2O are plotted on a ternary diagram. A typical weathering trend for sediment parallels the A-CN line, where the sediment loses Ca, Na, and K as it becomes more weathered from the original source rock. The level of K-enrichment, reflecting K-metasomatism and possible amorphous silica by-product, can be estimated by projecting each sample plotted on the A-CN-K ternary diagram back to the assumed, pre-metasomatized, original position (CIAcorr) along the weathering trend line (A-CN parallel line)16,46,47,48. The level of diagenesis experienced in each well is calculated by determining the difference between CIA and CIAcorr for each sample using the A-CN-K plot. Samples from each well were plotted on an A-CN-K plot. For the Kaybob well, XRD data was used to determine which samples should be included, since the A-CN-K plot is only used for analyzing weathering and K-enrichment in siliciclastic sediments. Therefore, only the lower (3304.36–3333.25 m) and upper Duvernay (3346.15–3359.13 m) were included in the analysis and the middle Duvernay (3333.25–3346.15 m) samples were excluded as these are almost entirely composed of carbonate.

Short- (SWIR; 970–2510 nm) and longwave (LWIR; 7400–12,100 nm) infrared reflectance spectra of the three Duvernay cores were collected at the University of Alberta using imaging spectrometers part of commercial SisuROCK systems. Cores boxes, containing slabbed Duvernay cores marked with intervals of spot samples taken for geochemistry, were scanned at a spatial resolution of 0.8 (SWIR) and 0.85 (LWIR) mm/pixel. We predicted TOC and Al2O3 in core imagery using spectral models developed for shales as detailed in Rivard et al.28 The models are based on spectral attributes (e.g., absorption features) related to mineralogy or TOC. The development of the models involved four steps: (1) matching core intervals sampled for geochemistry and TOC to corresponding hyperspectral imagery to generate a representative reflectance spectrum per sample interval, (2) wavelet analysis to highlight mineralogical features in reflectance spectra, (3) use of correlation scalograms to select key spectral features predictive of TOC and geochemistry, and (4) regression analysis to generate predictive models. A predicted weight percent value (TOC and Al2O3) is computed for each pixel and results for the core are displayed as greyscale images (Fig. 2). We also estimate the relative abundance of opal-A in core imagery. Amorphous SiO2 is readily distinguished from crystalline SiO2 in the Duvernay where amorphous SiO2 has a single reflectance peak near 9000 nm and crystalline SiO2 has a double reflectance peak near 8400 and 9200 nm28. The difference in the reflectance spectra is caused by Si–O–Si stretching in crystalline quartz as compared to amorphous quartz30,49. An opal-A index was designed to measure the relative strength of the two SiO2 features in the spectra of each image pixels and compare changes across the image. The strength of the features was measured from continuum removed spectra. Specifically, the opal-A index was computed as (CRa-CRb)/(CRa + CRb) where CRa and CRb are respectively the average continuum removed emissivity from 8312–8493 nm and 9017–9206 nm. The carbonate-rich and clay-rich pixels were excluded for this calculation. The index value is computed for each pixel and displayed as a greyscale image for the core. (Fig. 2).

SiEX, and δ13C(org) plotted versus depth; LWIR hyperspectral images of detected opal-A, and predicted Al2O3, and TOC (LWIR; greyscale). The dashed line refers to 0% SiEX. Left, middle, and right panels correspond to Kabob, Willesden Green and East Shale Basin respectively. The greyscale images have minimum and maximum image line values for opal-A index, %Al2O3, and %TOC of: 0.0–5.2, 0.0–19.5 (15.2), 0.0–4.2 (4.35) (Kaybob); 0.0–5.4, 0.0–15.2 (15.9), 0.0–5.5 (13.2) (Willesden Green); 0.0–4.3, 0.0–18.3(11.4), 0.0–4.1 (9.8) (East Shale Basin).

All oxide, δ13C(org), and TOC values for the three wells analysed in this study are detailed in Table 1.

Values of SiEX20 that were calculated using Eq. (1) are plotted versus depth to show the spatial distribution of SiEX in the three Duvernay wells (Fig. 2). Values below zero indicate that all of the SiO2 in the sample is associated with Al2O3, which is used here as a proxy for shale content. Samples from the ESB core, which contains samples from the Duvernay, shows only minor SiEX, with an average calculated value of -0.45%. Average values of SiEX in the Kaybob and WG wells are 12.51% and 6.00%, respectively, and average values for the Duvernay in Kaybob and WG are 17.37% and 10.94%, respectively, with SiEX absent in the middle carbonate in the Kaybob well. A sample from the Kaybob well shows the highest value of SiEX 50.37% and the maximum value in the WG well is 39.01%. δ13C(org) ranges from − 29.835 to − 25.709‰, − 30.057 to − 27.099‰, and − 30.248 to − 25.945‰ in the Kaybob, WG, and ESB wells, respectively.

Figure 2 also displays hyperspectral images produced for opal-A content, Al2O3, and TOC. Al2O3 is a clay proxy and shows an overall negative correlation to elevated SiEX. Intervals of elevated TOC correspond to increased SiEX in the Kaybob well, but in the other two wells there appears to be minimal correlation between these two components.

Thin sections from the intervals of enriched amorphous SiO2 detected by the hyperspectral analysis, contained evidence of biogenic SiO2. SiO2 was detected as large (~ 30 μm diameter; Fig. 3) rounded to sub-rounded, spherical to sub-spherical particles; some with spiky projections.

Thin section photos from the Kaybob well (04-19-064-22W4). All sections were polished, impregnated with blue epoxy to highlight porosity, and half stained with Alizarin red to detect calcite and aragonite content. (A) Depth 3309 m (upper Duvernay), plane polarized light, SiO2 (yellow arrows, down), and CaCO3 (green arrows, pointing up). (B) Same as A in crossed polarized light. (C) Depth 3309 m (upper Duvernay), plane polarized light, SiO2 (yellow arrows, down), CaCO3 (green arrows, up), possible radiolarian (blue arrow, diagonal). (D) Same as (C) in crossed polarized light.

Principle component analysis (PCA) used to analyze reported oxide values (Fig. 4) and TOC reveal that in the Kaybob well, SiO2 correlates with P2O5, TOC, and Na2O, in e1 versus e2 eigenvectors (Fig. 4a). When a third vector (e3) is plotted against PC1, SiO2 correlates with clay indicator oxides (e.g. Al2O3, TiO2, K2O; Fig. 4b). In the WG e1 versus e2 plot SiO2 correlates to P2O5 and TOC and e1 versus e3 plots show SiO2 more closely associated with the main group of clay-associated oxides (Fig. 4c,d). SiO2 is consistently associated with clay indicators in the ESB well (Fig. 4e,f).

PCA analysis of Duvernay Formation oxides in (A,B) Kaybob, (C,D) Willesden Green, (E,F). East Shale Basin. Al–Fe–Mn ternary plot inserts in (A–C) determine whether element chemistry infers a possible hydrothermal source (blue) or non-hydrothermal (yellow)(Adachi et al42.).

Al–Fe-Mn plots for Kaybob and WG wells show samples plotting outside of the field corresponding to a hydrothermal Si source, as defined by Adachi et al.42. Nine ESB samples plot inside this field, and the rest in the same region as the WG and Kaybob samples (Fig. 4).

The level of diagenesis (K-metasomatism) experienced in the Duvernay in each well, as determined by CIA-CIAcorr, ranges from 9 to 17 for ESB, 14–17 for WG, 7–12 for the lower Duvernay in Kaybob, and 3–10 for the upper Duvernay (Fig. 5).

A-CN-K (A12O3–CaO* + Na2O–K2O) plots of Kaybob well (A) and XRD data versus depth (B). The chemical alteration index (CIA) is used to find the level of diagenetic alteration the samples have experienced44,45,46,47,48. Samples are plotted back to the weathering trend along a diagenesis line (from K2O), to the weathering trend and over to the CIA (CIAcorr)46 to correct for K-enrichment. The CIA-CIAcorr of the sample determines the level of K-enrichment for each sample. The highest values of CIA-CIAcorr are noted here for each well.

The punctata Event (pE) was first recognized by a large positive δ13C excursion (up to 4.5‰) in the Frasnian punctata (conodont) Zone in Poland, Czech Republic, and China, but did not seem to correlate to any major sea level change or climatic fluctuation4. A key change was taking place in the terrestrial environment, however, the proliferation of aneurophyte and archaeopterid progymnosperm forests1 resulted in deeper and more complex root networks and increased pedogenesis2,7,8. This transition marks a significant shift in the interaction between the lithosphere and the hydrosphere. Pedogenic weathering introduced higher levels of terrestrially-derived nutrients (e.g. P) into the marine environment resulting in increased primary productivity, and oxygen stratification8. The burial of organic carbon associated with bottom water anoxia combined with increased upper water column productivity elevated δ13C ratios50, resulting in several positive excursions seen from the late Devonian through to the Carboniferous1,8.

In this study, positive δ13C(org) isotope excursions (2.9–3.7‰) are present near the base of the Duvernay Formation (Fig. 2). The isotopic shift at the base of the Duvernay/Perdrix was first recognized in the WCSB in the subsurface by Holmden et al.51, and in the Rocky Mountains in Alberta by Śliwiński et al.9, who recognized it as the pE. Śliwiński et al.18 Fig. 7 reveals two δ13C(org) excursions, one in the Maligne Formation (Majeau Lake Formation equivalent), thought to be a side effect of dolomitization, and one in the Perdrix Formation (Duvernay Formation equivalent). The positive shift in the Perdrix is Events III of the pE and the abrupt return to background values is Event IV (Fig. 6), according to comparison of recently updated Frasnian conodont stratigraphy in the WCSB32, and correlations between numerous locations worldwide in Pisarowska et al.12 In this study, the Kaybob well displays two shifts, one larger positive excursion in the Waterways Formation of the Beaverhill Lake Group through into the Majeau Lake Formation, and another in the lower Duvernay. Śliwiński et al.18 also records this positive shift in the Majeau Lake/Maligne interval but recommends caution in its interpretation, since this interval contains dolomitized strata. Although, dolomitization is not usually considered as likely to affect δ13C(org) isotope data and this positive perturbation is present in the Kaybob well of this study at the same interval implying that this interval might be worth further investigation, despite its divergence from the worldwide records. The WG, Kaybob, and ESB wells each display a δ13C(org) excursion in the lower Duvernay, of which the base, positive excursion is considered to be Event III, and the return to background values, Event IV12 (Fig. 6). The slight offset of these excursions is not surprising considering that the timing and rate of extensive plant colonization and development would be staggered throughout the globe8. The δ13C(org) excursion in the Kaybob well, which is the most distal of the three wells included in the basin transect has the weakest Event IV δ13C(org) perturbation (1.5‰) and the ESB well, which is the most proximal and deposited on a pre-existing carbonate shelf (Cooking Lake Formation), displays the strongest perturbation (3.7‰). This shift in the magnitude of δ13C(org) values across the basin follows the model proposed by Śliwiński et al.18 that found the magnitude of the pE δ13C(org) excursion for Event III is reduced from shelf to basin. With Event IV of the pE identified in each well, the LWIR data and SiEX was compared to δ13C(org) data to determine whether predicted increased productivity (plankton = radiolarians), correlate with the onset of forest expansion, increased soil generation and nutrient delivery, as predicted in Algeo and Scheckler2.

δ13C(org) records from Padberg, Germany12, Wietrznia, Poland52, western Canada (Miette Platform)18, and this study of the Duvernay Formation from western Canada. These records are plotted with the biostratigraphic column from Pisarzowska et al.12 (1 = revised conodont zonation in Pisarzowska et al.12 and 2 = Racki and Bultynck53) and Wong et al.31 (1 = Ziegler and Sandberg54 and 2 = Montagne Noire Zone55).

In the Kaybob core, there is a slight increase in SiEX following the largest δ13C(org) shift (3363.04 m; Event II; Fig. 2) followed by elevated SiEX and opal-A that corresponds to a second, smaller δ13C(org) shift (3352.00 m; Event III). Finally, there is a significant and sustained increase in SiEX and opal-A SiO2 in the upper Kaybob Duvernay (3331.45 m) that does not directly correspond to a δ13C(org) shift. In the WG core, SiEX and opal-A SiO2 content increases when δ13C(org) shifts to its most positive value (3274.48 m; Event III) and SiEX and opal-A SiO2 continues to be present throughout the lower Duvernary. In the ESB well, in a position farthest from the open ocean, on the shelf, and behind several large reef platforms, there is the largest of the Event III δ13C(org) shifts (2243.22 m) and another smaller shift up Sect. (2232.65 m) that corresponds to an increase in SiEX and slightly elevated opal-A SiO2. Relative to the other two wells, the ESB Duvernay contains minimal SiEX and opal-A SiO2, which is perhaps unsurprising considering it is located on the platform behind several large reef complexes. In each of the three datasets, it appears that the opal-A SiO2 negatively correlates with intervals of increased Al2O3, an indicator of clay input. Elevated levels of SiEX and opal-A SiO2 may be related to δ13C(org) shifts inferred to signal changes in the global carbon cycle linked to the pE; however, not all intervals of increased SiEX and opal-A SiO2 are correlated to the exact onset of these excursions. There may be other sources of opal-A SiO2 in the basin, besides radiolaria related to the pE, therefore, it is critical to interrogate the data and ascertain whether there could also be multiple sources of amorphous SiO2, especially given the lack of a δ13C(org) excursion in the upper Duvernay of the Kaybob well, where there is a significant amount of SiEX identified by the LWIR as opal SiO2.

The possible sources of amorphous SiO2 in shales or mudrocks, include: (1) a biogenic source14,24,38, (2) hydrothermal alteration42, or (3) a by-product of clay diagenesis (e.g. smectite-illite or illite to muscovite transitions)43,56,57.

PCA analysis (Fig. 4) of the Kaybob and WG wells show that SiO2 correlates with P2O5 and TOC in the e1 vs e2 (Fig. 4a,c). Higher values of P2O5 content indicate increased paleoproductivity9,14,58,59. The association of SiO2 with P2O5 and TOC may therefore suggest that SiEX in WG and Kaybob is primarily biogenically sourced. This is supported by Harris et al.38 where SiEX in the Duvernay is attributed to a biogenic source as intervals that are high in excess Si (> 5%) correlate to high TOC. Na2O was also associated with SiO2, P2O5, and TOC in this field. Na2O is typically considered a clay-proxy but Na2O values can be unreliable due to the incorporation of drilling fluids, so it is removed from our interpretation of all PCA plots in this study41. In PCA analysis (Fig. 4), e1 vs. e3 in the Kaybob well shows SiO2 associated with clay indicators (e.g. TiO2, Al2O3), suggesting that there is also a contribution of SiO2 from siliciclastics in this well, whereas the SiO2 in e1 vs. e3 in the WG well is not tightly clustered with clay indicators, but is also disassociated from P2O5 and TOC. This may indicate the presence of aeolian silt in the WG well since this type of silica would not necessarily be tightly correlated to clay proxies. A hydrothermal Si source can be discounted for the Kaybob and WG wells based on Al–Fe-Mn plots that reveal that these samples plot close to the Al field, indicating that they are not associated with a hydrothermal source (Fig. 4)42. Petrographic analysis of thin sections from the Kaybob Duvernay, specifically from the intervals (3309 m and 3350 m) of elevated SiEX and shown by the hyperspectral images to contain opal-A SiO2, reveal several rounded particles of SiO2, some that display lacey or porous structures suggesting that they are radiolarians (Fig. 3c,d). All the thin section photos shown in Fig. 3 were taken in the regions that were stained with Alizarin red. The round to sub-rounded, spherical to sub-spherical particles shown in Fig. 3 (green arrows, pointing up) may also be the source of the amorphous SiO2 shown in the hyperspectral images (Fig. 2). Schieber14,17 states that identification of quartz silt in shales may in fact be algal cysts or spores that are filled with diagenetic SiO2, sourced from biogenic SiO2 (radiolaria or sponges), similar to the rounded to subrounded SiO2 particles in Fig. 3. In the ESB well, all PCA analysis shows SiO2 correlated to clay indicators, suggesting that most SiO2 in the ESB well is sourced from siliciclastics. However, there are nine samples that plot close to the Fe field in the Al–Fe-Mn ternary plot, which is associated with metalliferous sediments interpreted as hydrothermal precipitates (Fig. 4e)42. These intervals do not correlate with increased levels of SiEX (Fig. 2) and overall, SiEX and LWIR imagery indicates that SiEX and opal-A in the ESB well is minor.

Burial clay diagenesis that may result in opal-A as a by-product of K-enrichment was evaluated as a potential source of SiO2 in the Duvernay wells. This was done by plotting oxide data on an A-CN-K ternary diagram44,45,46,47,48 to determine how far the samples have shifted away from the predicted A-CN parallel weathering line that predicts the loss of CaO, Na2O, and K2O, expected in chemical weathering of sediments. The distance away from this weathering trend, towards the K2O apex determines whether the sediments have experienced significant K-enrichment that is associated with the smectite-illite reaction, illitization or K+ enrichment (Fig. 5). Subsiding marine basins in lower temperature settings of 100–120 °C are likely to experience K-metasomatism that may result in smectite-illite transitions and precipitation of amorphous SiO2 as a by-product of the reaction43,45,60. From the A-CN-K plots it is evident that samples from the ESB and WG wells have experienced a similar level of K-enrichment, with CIA-CIAcorr values of up to 17 (Fig. 5c,d). Kaybob samples have experienced the least K-enrichment, with the upper Duvernay displaying CIA-CIAcorr values of only 3–10. Therefore, the least alteration was experienced in the upper Kaybob Duvernay, which also show the highest values of SiEX and amorphous SiO2 (Fig. 2). While there is no way to definitively determine how much amorphous SiO2 may be contributed as a by-product of clay diagenesis, Abercrombie et al.43 predicts that significant volumes of SiO2 would not be contributed through this process. Accordingly, while all the samples from the Duvernay wells have undergone K-enrichment through clay diagenesis, which may result in some contribution of amorphous SiO2, it is likely that the primary source is biogenic for amorphous Kaybob and WG SiO2 in particular, based on the association of SiO2 with P2O5 and TOC in the PCA analysis (Fig. 4).

In past studies of the pE, like many other major shifts in climate and the carbon cycle, δ13C(org) or δ13C(carb) excursions are used to pinpoint the onset of a given event (e.g. large igneous province eruptions). During these shifts, certain associated geochemical signals are expected in the rock record that we use as proxy for understanding the causal events that led to changes to the atmosphere, biosphere, or hydrosphere. Pisarowska et al.12 reviews possible causes for the pE excursion and points out that the IIc61 flooding surface corresponds to the onset of the pE in several of the locations it is recognized in and intensified water exchange between epeiric seas or and the open ocean coupled with the changes in the terrestrial environment are the most likely cause. The Alamo impact in southern Nevada62 and potential volcanic eruptions63,64, have also been cited as possible explanations for the excursion but Pisarowska et al.12 points out that the onset of these events does not correspond to the onset of the pE and major volcanic eruption would be more likely to result in a negative excursion, as seen at the Permian–Triassic extinction interval. In the WCSB, reef building continues through this interval, implying that this marine life persists through this interval at this location.

With the cause of the pE considered and attributed partially to the development of deeper root networks and thicker soil horizons during the Middle to Late Devonian, it is likely that riverine discharge carrying increased concentrations of liberated P8 would increase along with planktonic biomass. With increased productivity and upwelling predicted through models and use of proxies2,9, an increase in biogenic SiO2 reflecting the presence of opal-A radiolarian tests at the onset of the pE may be expected. All three wells, even the ESB well display increases in opal-A at or shortly after the onset of pE Event III and based on the PCA, Al–Fe–Mn, and A-CN-K analysis presented here, the source is likely biogenic. However, the concentrated opal-A and TOC present in the Kaybob upper Duvernay, which has the lowest overall degree of K-enrichment and no evidence of hydrothermal influence, does not correspond to a δ13C(org) excursion, despite satisfying criteria normally used to explain positive δ13C shifts. The significant increases in biogenic SiO2 and TOC have been attributed to upwelling during a second-order sea-level rise, unrelated to the pE38. However, increased concentrations of biogenic Si and TOC are not observed in the WG well. The distribution of opal-A in the Kaybob well agrees with established models of Duvernay deposition whereby continentally derived material is deposited in east–west clinoforms34. In the WG basal Duvernay there is a slight increase in biogenic Si coupled with weathered clay, and possible quartz silts (WG PCA e1 vs. e3). With westward-thinning clinoforms, the Kaybob well, which is closest to the open ocean, is more favourable for deposition through suspension, favouring preservation of siliceous skeletal material. Overall, the Kaybob well does contain less clay and more opal-A SiO2 than the ESB well. The conundrum illustrated in this study is that the δ13C(org) excursions signalling the pE do not necessarily correspond to the most geochemically and mineralogically distinct interval that would seem to fit the predicted response of increased productivity and preservation of organic matter (e.g. high concentration of biogenic Si and TOC in the upper Duvernay in Kaybob). The expected response of increased productivity and preserved organic matter may be diluted in the basal Duvernay at the pE due to the accompanying increase of terrestrially-derived sediments associated with deeper root networks developed during the pE. Offsets between major climate shifts signalled by δ13C and the response recorded in sedimentary strata have been identified in other studies of the punctata Event8 and intervals of other significant climatic events or potential biodiversity gaps (e.g. Cambrian SPICE Event64; Paleocene-Eocene65). One thing is certain, many different methods of investigation should be used when interpreting δ13C excursions and the onset of significant climatic shifts, versus the changes (e.g. increased productivity) that are recorded in the rock record. The links between these are still not well understood. As observed in this study and Śliwiński et al.18, the magnitude of δ13C excursions also varies, depending on the position in the basin. The effects of major shifts in δ13C records (e.g., changes in productivity, increased TOC, etc.) are also variable in ocean basins and their position in the sedimentary record. Given the importance of shifts in weathering, productivity, and the biogeochemical Si cycle around major climate shifts and extinctions, separation of amorphous versus crystalline SiO2 is critical to future studies of these events16,27. The use of LWIR in this study is the first attempt at mapping amorphous and most likely biogenic SiO2 in an ancient basin. Further studies using this technique may help us understand biogeochemical silica cycling and detect significant shifts in this cycle through geological time. Current methods of differentiating SiO2 polymorphs have many limitations27. Early plant colonization of landscapes in the Middle to Late Devonian increased delivery of P to the marine environment, likely increasing productivity in the upper water column and localized bottom water anoxia, but with eventual stabilization of P liberation as the vegetation becomes established, it is unknown whether these shifts cause sustained basin-wide changes in the marine environment8. Mapping intervals of increased productivity (biogenic SiO2) on a basin scale will aid our understanding of terrestrial-marine teleconnections during periods of significant climatic shifts.

This study examines the distribution of amorphous SiO2 in the Duvernay Formation in three wells across a basin transect to determine whether an increase in productivity (e.g. radiolarians) is associated with the emergence of deeper root networks and thicker soil horizons contributing to terrestrially-derived nutrients in the oceans during the punctata Event (pE). Previous investigation of modern and ancient records for intervals of amorphous SiO2 that may be attributed to biogenic sources have relied on geochemical proxies or separated aliquots that have not captured all of the amorphous SiO227. LWIR hyperspectral imaging allows us to detect opal-A SiO2 on a macroscale, as a continuous dataset, using a non-destructive instrument. This ability to quickly and efficiently detect intervals of opal-A SiO2 means that we may start to map these intervals on a basin scale to improve paleogeographic reconstruction and map past oceanic circulation. Opal-A SiO2 identified by LWIR imaging was determined to have two sources: 1) a by-product of clay diagenesis identified using an A-CN-K plot, and 2) biogenic silica, which is the primary source of SiO2 in two of the three wells. In the case of the Duvernay in the Western Canada Sedimentary Basin, the effect of the pE resulted in increased paleoproductivity in two areas of Duvernay deposition basinward of a large barrier reef. The third area represented by the ESB well, which is located on the carbonate platform and behind several large elongated barrier reefs, shows only minor amorphous SiO2. While the A-CN-K plot suggests there may have been a minor contribution of SiEX or opal-A SiO2 from clay diagenesis, the basinward increase in SiEX and opal-A SiO2, correlation of SiO2 to P2O5, TOC, and the δ13C(org) shift associated with the pE, and detection of possible radiolarian tests or SiO2-filled algal cysts all point to a dominantly biogenic SiO2 source in the Duvernay. This finding supports theories that oceans experienced an increase in productivity as the world’s forests expanded and developed deeper root networks which increased soil genesis and delivery of terrestrially-derived nutrients into the marine realm. However, the highest concentration of SiEX, biogenic opal-A SiO2 and TOC is in the upper Duvernay of the most distal well and does not correspond to a δ13C(org) excursion. This study demonstrates the need for more studies that examine the correlation of δ13C excursions and reactions recorded in the sedimentary record and the need for differentiation of amorphous from crystalline SiO2 so that we may better understand global silica cycling through terrestrial-marine teleconnections.

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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The authors would like to express their gratitude to the Alberta Energy Regulator’s Core Research Centre and John Pawlowicz and Dean Rokosh for their assistance in data collection. MacEwan University is also thanked for its support of this work through a project grant. The work has also been supported by the Natural Sciences and Engineering Research Council of Canada (Grant Nos. RGPIN-2021-02785, RGPIN-2020-1506) Discovery Grant Program through grants awarded to Drs. Hilary Corlett and Benoit Rivard. The authors would also like to thank two anonymous reviewers and Editorial Board Member Dr. Di Yang, who offered carefully considered suggestions that were incorporated into this manuscript, thereby improving the overall quality of the study.

Department of Earth Sciences, Memorial University of Newfoundland and Labrador, St. John’s, NL, A1B 3X5, Canada

H. Corlett

Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB, T6G 2E3, Canada

J. Feng & B. Rivard

Alberta Geological Survey, Alberta Energy Regulator, Edmonton Regional Office 402, Twin Atria Building, 4999 - 98 Avenue, Edmonton, AB, T6B 2X3, Canada

T. Playter

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The corresponding author was responsible for conceptualization, methodology, data collection and analysis, and writing the original draft. Authors B.R. and J.F. were responsible for methodology, data collection, analysis, and validation, and writing the original draft. Author T.P. was responsible for data collection and review of the original draft.

Correspondence to H. Corlett.

The authors declare no competing interests.

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Corlett, H., Feng, J., Playter, T. et al. Mapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification. Sci Rep 13, 1516 (2023). https://doi.org/10.1038/s41598-023-28542-y

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Received: 29 July 2022

Accepted: 19 January 2023

Published: 27 January 2023

DOI: https://doi.org/10.1038/s41598-023-28542-y

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Mapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification