Volcanogenic Nutrient Fluxes and Plant Ecosystems in Large Igneous Provinces: an Example From the Columbia River Basalt Group
By Jolley, David W Widdowson, Mike; Self, Stephen
Abstract: Research from biological and geological sources has highlighted the role of volcanoes in the outgassing of P, and thermal fixation and subsequent atmospheric oxidation of NO^sub x^ in volcanic environments. The impact of these nutrient fluxes on biological systems has been demonstrated on present-day Hawai’i, and here we consider the impact on the plant communities within a large igneous province (LIP). The Miocene Columbia River Basalt Province, the youngest LIP on Earth, contains many sedimentary interbeds between the flows of the major extrusive phase, and these interbeds preserve variable but often diverse palynofloras. By integration of palynofloral analysis with analysis of macronutrient levels in the interbeds it can be suggested that there may have been significantly elevated levels of P within the lava field proximal to the source vents, a distribution mirrored by Ca and Mg. Evidence for potential volcanogenic eutrophication is restricted to >10^sup 3^ year duration interbeds, contemporary with eruptive activity elsewhere in the LIP. The geochemistry and palynology of other interbeds demonstrate nutrient deficiency, with a potential route to nutrient sufficiency available from long-term symbiotic N-fixation. Elsewhere within the Columbia River Basalt Province, this process is short- circuited by the imput of felsic ash from the nearby Cascades Range of volcanoes.
The environmental impact of large igneous province (LIP) volcanism is an issue of considerable debate and investigation, because the release of prodigious quantities of volcanogenic CO2, SO^sub 2^ and NO^sub x^ provides ancient analogues to modern pollution effects. Recently, studies have focused on the role of volcanism in climate forcing, in terms of both atmospheric warming via CO2 pollution (Stevenson et al. 2003) and cooling via SO^sub 2^ advection (e.g. Gauchi & Dise 2002; Jolley & Widdowson 2005; Self et al. 2006). Other work on volcanic terrains has concentrated on the eruptive processes and their environmental consequences via aerosol production on a regional scale (e.g. Grattan & Pyatt 1994; Highwood & Stevenson 2003; Stevenson et al. 2003; Thordarson & Self 2003; Chenet et al. 2005; Oman et al. 2006).
The current work focuses upon the remains of plant ecosystems from the Miocene (c. 15 Ma) Columbia River flood basalt province, where the lavas constitute the Columbia River Basalt Group, the youngest LIP on Earth. Inter-lava sedimentary rocks within the Columbia River Basalt Group represent quiescent periods between eruptive phases, and are often characterized by plant fossils (Fig. 1), which become more common as the duration of the interflow increases. Here, we use a combination of field and laboratory data to evaluate not only the potential for large basaltic lava-forming volcanic eruptions to disrupt and devastate ecosystems on a regional or global scale (Wignall 2001; White & Saunders 2005), but also their potential to modify nutrient flow into associated ecosystems. The ability of complex ecosystems to coexist with the apparently hostile environments generated by LIP flood basalt eruptions has been a polemic issue (Jolley 1997; Ellis et al. 2002). Here, we suggest that the deposition of nitrates and phosphates derived from volcanic gases, and the readily available supply of element nutrients derived from the weathering of volcanic products and throughput of meteoric waters, may provide conditions that favour the development of high-productivity ecosystems during the eruptive lifetime of an LIP, and we provide some supporting evidence.
To understand how the biological system responded to Columbia River Basalt Group flood volcanism, it is first necessary to briefly outline the dynamics of lava flow emplacement, and the evolution of the lava pile in a typical LIP (Self et al. 1998, 2006). Single eruptive events typically produce >10^sup 2^ km^sup 3^ of lava, and single eruptive units of >10^sup 3^ km^sup 3^ are not uncommon. These eruptive units consist of laterally extensive lava flow fields that evolve continually over periods of years to decades during each eruption. During the development of the lava field, areas both proximal and distal to the source vents will have hot lava surfaces that may fix gases in air (see also Mather et al. 2004a) and release gases according to changes to the supply of new lava from the vents to the propagating tips of lava flows. In this manner, areas of the lava field may become periodically abandoned, and new hot lava will be fed elsewhere onto the flow-field over the period of eruption. This semi-random pattern, largely controlled by the topography onto which the lava field is being emplaced, will be repeated every time there is an eruption over areas of 10 000 km^sup 2^ during the LIP’s lifetime. These changes result in a highly dynamic physical environment across the surface of single eruptive units, and across each entire active flow field, and will have profound effects upon the nature and duration of development of coeval ecosystems. An onset and cessation of this dynamic environment is repeated each time there is an eruption in the lifetime of an LIP.
Once a flood basalt eruption event ceases, the whole of the surface of the eruptive unit will become available for colonization, but because of the constructional dynamics, some parts of this surface will have already experienced periods of volcanic quiescence, even during the eruptive event. In many instances exposure of the newly formed lava surfaces will permit the development of weathering horizons, or else they will receive ash fallout or transported sediment that accumulates to form interbeds, or both. Although some interbeds may represent periods of prolonged quiescence at a particular locality, it does not necessarily follow that the evolving flood basalt province is dormant in its entirety during the formation of each interbed.
Nutrient flow in present-day volcanic environments
Detailed studies of long-term macronutient availability within active lava fields are few, most records being focused around the initial processes of biotic reassembly after an eruption (Whittaker et al. 1989; Thornton 2000). These studies have demonstrated the limitations imposed by the lack of macronutrients (N, P, K, Mg, Ca) on the earliest phases of post-eruption biotic reassembly. In the case of island volcanoes, the importance of strand-line detritus as a source of extrinsic nutrient input is paramount (e.g. on Surtsey and Krakatau, Thornton 2000), but this vector would have been of minor importance in the larger scale Columbia River province, which lacks significant marine margin, and would have resulted in slower recolonization (Harrison et al. 2001).
Away from the coastal belt, or in intracontinental regions, the slow accumulation of nutrients from aerial plankton (Thornton et al. 1988) is succeeded by lichens (in particular, Slereoculon species that degrade mafic minerals; James et al. 2000; Crews et al. 2001), then ferns with accompanying trapping of detritus and resultant soil formation. Establishment of later succession communities can be dependent on climatic and elevation factors. On Surtsey, the subarctic climate slows weathering, whereas on Etna and Motmot (Harrison et al. 2001), precipitation limits plant growth. The rapidity of community succession is also influenced by the resource from undamaged reservoirs of species outside the limits of the lava field (e.g. Mount St. Helens; del Moral & Wood 1993).
Apart from this small body of research, little consideration has been given to the processes of biotic succession on developing lava fields, especially large-scale ones such as flood basalt provinces, over periods of 103-106 years duration. It is fortunate that the Hawaiian islands have provided an experimental laboratory on which scientists have been able to test and observe processes over these longer time periods. Data and observations from these studies provide a comparative system, which can be used to assist in the interpretation of soil profiles (Sheldon 2003), plant successions, and nutrient accumulation or loss in some large igneous provinces.
P, Mg, Ca
Observations from Hawai’i have provided evidence (Vitousek 2004) that P becomes bio-available as P^sub 4^O^sub 10^ from volcanic outgassing, being deposited downwind of active eruptions (Yamagata et al. 1991). P availability is also added to by lava flows that reach the sea, causing further fluxes to nearby regions. Both these sources of additional P flux make an important contribution to nutrient availability in early succession sites. Many sites in volcanic terrains are rich in P derived from these sources, with P levels significantly enhanced by weathering and breakdown of soil minerals. With increasing age and biomass, P becomes depleted on volcanic soils and this eventually presents the major limitation to plant growth (Vitousek 2004).
Base cations and P are all readily available as products from the breakdown of basaltic lava and ash in lava fields, other than in arid or cold regions. These inputs are fundamentally reduced after c. 100000 years (Vitousek 2004) by >50%. After this, plant communities, mainly mature forest on Hawai’i, receive up to 50% of the P budget from airborne dust in a manner comparable with tropical rainforest on the same latitude. These nutrientdepleted climax communities and associated soils are common in the oldest Hawaiian profiles, where strontium (Sr) depletion can be used as a proxy for the availability of Ca, Mg and P. In this system, almost total removal of volcanogenic Sr isotopes is achieved within 10^sup 4^ years, forcing plant nutrient dependence on atmospheric P deposition. N availability
Studies of early colonization by plants on lava fields have identified the crucial role played by low N availability (e.g. Walker & Aplet 1994). N is available in the initial phases of recolonization from aerial plankton (Harrison et al. 2001), or by decomposition and burning of already established biomass (DeBano & Conrad 1978). Plants in early successional communities growing on lava fields could therefore be expected to have low N demands, or to be symbiotic N-fixers. However, an alternative source of N in primary volcanic landscapes has recently been identified. Working on active volcanoes, Mather et al. (2004a) demonstrated NO^sub 3^ production at the interface between hot lava and the atmosphere, the NO^sub 3^ subsequently being oxidized to NOx. Mather et al. used SO^sub 2^ production as a proxy for measuring the production of nitric acid, but could not readily quantify the NO^sub x^ volumes produced. Working in a different area, Heath & Heubert (1999) and Heubert et al. (1999) had previously identified thermal fixing of NO^sub 3^ as a method of transfering and enhancing N availability to lava-field plant communities. Measurements taken on the island of Hawai’i in volcanic fog clouds (‘vog’) recorded NO^sub 3^ levels at 220 ppb, orders of magnitude above background levels (Heubert et al. 1999). Over a time frame of hours to days, this NO^sub 3^ oxidizes to NO^sub 2^ and HNO^sub 3^, species that are easily deposited either as cloudwater or later by dry deposition. This amounts to a contribution of 2.4 kg N ha^sup -1^ year^sup -1^ into a N- depauperate environment. On Hawai’i, the distribution and efficiency of NO^sub x^ deposition is controlled by local wind directions. The observation by Vitousek (2004) and coworkers that the level of N deposition is unusual, because Kilauea has been active nearly continuously since 1983, is not applicable in the LIP context, where volcanism would have been on a larger scale and long-lasting. Thermal fixation thus provides a source of N, which may have been of significance to the primary colonization of lava flows by plants.
Estimating NO^sub x^ flux from LIP lava-field eruption
During volcanic activity, generation of biologically available nitrogen (N) occurs through thermal effects: through the interaction between atmosphere and magma lakes or hot ejecta plumes, by lava surface cooling, or by volcanically induced lightening within ejecta plumes (Mather et al. 2004a, b; Martin et al. 2006; Mather & Harrison 2006). Its efficiency is highly temperature T-dependent, with NO formation typically beginning above 800 [degrees]C, and generating increasingly significant quantities above 1000 [degrees]C. The resulting NO is then converted to HNO^sub 3^ in the atmosphere over periods of hours to days via a number of reaction schemes. Because a high-temperature lava-atmosphere or magma- atmosphere interface is required for significant NO^sub x^, and hence HNO^sub 3^ production (Mather et al. 2004a), NO^sub x^ fixation is more probable in instances where hotter eruption products are generated (i.e. basaltic eruptions; see Martin et al. 2006). A relationship linking amounts of magma released in a volcanic eruption to the amount of fixed N produced (and thus amounts of NO^sub x^) has been established (Mather et al. 2004a).
A numerical relationship relating the quantity of SO^sub 2^ release in a volcanic plume to that of NO^sub x^ has been established (Mather et al. 2004a). Importantly, quantities of volcanogenically generated SO^sub 2^ and volcanically fixed NO^sub x^ release may be described in terms of a simple mass ratio relationship (HNO^sub 3^/SO^sub 2^ = 0.02-0.07; Mather et al. 2004a), indicating an interplay between the extent of available hot magmatic or lava oxidation surfaces (i.e. lava lakes, fire fountaining behaviour, gas or ash plumes, or lava lakes) and the magma composition and/or eruptive behaviour. Those magmas that generate the hottest, most fluid, and hence aerially extensive lavas offer the greatest potential for developing NO^sub x^ oxidation surfaces. Low-viscosity magmas such as these are typically mafic in composition, and such compositions commonly produce fire- fountaining behaviour, the development of lava lakes, and flow ‘skylights’ where atmosphere and erupted magma can interact directly. Moreover, these mafic magmas also contain higher concentrations of dissolved sulphur. In contrast, more acidic magmas, although potentially more explosive, contain low quantities of dissolved sulphur (Scaillet & MacDonald 2006), produce areally restricted lava flows, and have a significantly lower eruption temperature: in these instances, the controlling factors will serve to significantly reduce the NO^sub x^ fixation at non-basaltic eruptive events. Thus, the potential for the most prodigous NO^sub x^ production is likely to be associated with the eruptive style exhibited by LIP-type eruptions, and will be significantly enhanced in highly explosive (e.g. phreatoplinean) basaltic eruptions (Jolley & Widdowson 2005), and also by flow disruption processes such as those that give rise to a-a and rubbly pahoehoe-type lava flows (Gilbaud et al. 2005).
Mather et al. (2004a) established an empirical relationship between the quantities of SO^sub 2^ and NO^sub x^ in a volcanic plume (i.e. HNO^sub 3^/SO^sub 2^ = 0.02-0.07), providing a basis for estimating NOx production during ancient eruptive events. Given that a typical LIP basaltic magma probably contained 1000-1500 ppm S (Thordarson & Self 1998, 2003; Self et al. 2005, 2006), each cubic kilometre of erupted lava will contain (2.4-3.6) x 10^sup 9^ kg of S. Thus, using the established HNO^sub 3^/SO^sub 2^ relationship, a total of 1-5 x 10^sup 8^ kg (c. 0.1-0.5 Tg) of HNO^sub 3^ would be produced. Importantly, this estimate is consistent with the theoretical thermodynamic relationship of N fixation of 2 x 1010 mol km^sup -3^ (i.e. 12.6 x 10^sup 8^ kg HNO^sub 3^ for each cubic kilometre of erupted lava) suggested independently by Mather et al. (2004b). Accordingly, for ease of calculation the estimated value of 5 x 10^sup 8^ kg (0.5 Tg) of HNO^sub 3^ is adopted.
To understand how these amounts of NO^sub x^ might affect nitrification in the surrounding environment, such as sterile volcanic landscapes, it is next necessary to determine probable N deposition rates. Assuming that 1 km^sup 3^ erupted volume produces a c. 100 km^sup 2^ lava field (i.e. an average flow thickness of 10 m), and that the area affected by N-deposition is up to a magnitude larger than the lava field, then the N-deposition resulting from a 1 km^sup 3^ erupted volume will affect a total area of 1000 km^sup 2^. If all the NO^sub x^ fixed were then uniformly distributed the affected area, it would result in a deposition of 11 x 104 kg N km^sup -2^ for every cubic kilometre of basalt erupted (i.e. c. 1100 kg ha^sup -1^). At eruption rates of >10-100 km^sup 3^ a^sup -1^, large contintental flood basalt (CFB) eruptions (i.e. >1000 km^sup 3^) would require several decades to generate the associated lava fields (Thordarson & Self 1998; Self et al. 2006), and so the annual rate of N deposition becomes c. 100 kg ha^sup -2^. However, the process of volcanogenic NOx production and subsequent N deposition is not efficient, further reducing availability. For instance, if only 10% of the potential volcanic N fixation were returned to surrounding regions, it would result in a deposition of 10 kg N ha^sup -1^: Such an estimate is consistent with values of 8-22 kg N ha^sup -1^ measured across Hawaiian lava fields (Heath & Heubert 1999). From these estimates, it would appear that production of volcanogenic NO^sub x^ would have exceeded deposition by an order of magnitude. Levels of bioavailable NO^sub x^ would therefore be controlled by the efficacy and variability of the depositional process, rather than showing a close relationship to the volume of NO^sub x^ produced from volcanogenic thermal fixation.
Nutrient flux in LIPs
Interflow sediments contain records of the vegetation that colonized the lava-field surface, comprising macrofossil remains and abundant fossil pollen (Boulter & Kvacek 1989; Jolley 1997; Jolley & Bell 2002). The composition and distribution of pollen assemblages within intra-lava sediments is controlled by depositional environment and taphonomy. Within thicker interbeds accumulated over longer periods, pollen profiles can be used to trace the succession of plant communities from first colonization to destruction by volcanism. Analysis of these pollen profiles provides evidence of the maturity of the interflow profile, or increasing community complexity. Thus, identification of palynofloras from early successional communities indicates either immaturity of the sediment profile, or single or repeated environmental disturbance events. Late successional and mature communities occur less commonly within LIP lava fields, particularly in the zone proximal to the eruptive source, reflecting the frequency of lava eruptions.
Accordingly, we have examined a series of geographically and temporally separate sites within the Columbia River Basalt Group to inform the relative importance of possible N input pathways in large- scale eruptive events. In addition to palynofloral analysis, element ratios from sedimentary successions within the Columbia River Basalt Group are employed to determine the availability of nutrients at different localities and time horizons within the evolving LIP. These data are then synthesized to assess the impact of nutrient availability on coeval ecosystems during a major LIP eruption history. Based upon a synthesis of ^sup 40^Ar/^sup 39^ Ar dating of key CFBs (e.g. Deccan, Siberian Traps, Karoo and Parana) there appears to be a c. 1 Ma period during which the vast proportion of the total output is erupted (e.g. Baksi 1988; Baksi & Farrar 1990; Vandamme et al. 1991; Self et al. 2006). It is this high-output pulse that is of greatest environmental significance, and is likely to result in the highest NO^sub x^ fixation around eruptive centres. For the Columbia River the main pulse (>90%) produced the Grande Ronde Basalt and Wanapum formations (Fig. 2; Baksi 1988; Tolan et al. 1989; Reidel & Tolan 1992), lasting <1 Ma. Sedimentary interbeds from the high-output stage were sampled for the initial phase of this study, with the emphasis placed on those deposited in fluvial drainage systems. Analysis of sedimentary interbeds from other LIPs (Jolley 1997; Ellis et al. 2002) has shown that preservation of palynomorphs is favourable under such depositional conditions. Accordingly, we selected several sedimentary successions at varying distances from the contemporary eruptive source (Fig. 1), from within 150km from the fissure eruption sites to >300 km distal sections to the west near the edge of the province. These sections are from four stratigraphical intervals; the oldest is within the Grande Ronde Fm N2 subdivision, with two sites at, or equivalent to, the Vantage Interbeds between the Grande Ronde N2 subdivision and the overlying Wanapum Basalt Formation. A further interbed horizon was sampled from within the Wanapum Basalt Formation, between the basalts of Sand Hollow and Silver Falls, and the youngest interbeds were sampled at two locations between the Frenchman Springs and Roza members (Fig. 2).
In analysing the geochemistry of the Columbia River Basalt Group interbeds studied here, we have used two principal techniques. Standard XRF techniques have been used to analyse the full range of major oxides and trace elements present. Because of the time taken in preparing samples for XRF analysis, and the number of samples for analysis, a second suite of techniques was first tried in parallel. These techniques are more usually associated with environmental science applications, but have proven here to have produced a remarkably consistent dataset. Analytical results for Mg, Ca, Na, K, Ba, Sr and Al are closely similar for both XRF and flame atomic absorption spectrometry (FAAS; see below). Regression analysis of the Columbia River Al:base values, derived from both XRF and FAAS, yielded R^sup 2^ 80.5%, a trend reflected in regression analysis of single element concentrations.
XRF major and trace element analysis
Concentrations of major elements were determined by XRF at the Open University on fused glass discs manufactured from the fusion of one part rock powder (dried at 110 [degrees]C) with five parts of dried lithium metaborate-tetraborate flux (Johnson Matthey Spectroflux 100B) in Pt-5% Au crucibles at 1100 [degrees]C. Percentage loss on ignition (LOI) of volatile components (e.g. H2O, CO2, etc.) was determined separately by calculating weight loss after ignition at 1000 [degrees]C for 1 h. Analyses were performed using an ARL 8420+ dual goniometer wavelength-dispersive XRF spectrometer employing routine XRF procedures and analytical packages. Elemental intensities were corrected for background and known peak overlap interferences and medium-term instrumental intensity drift was taken into account using a drift normalization monitor. Large LOI values are common in carbonrich and clay-rich materials. To allow for direct comparison of elemental abundances within the current sample suites, the XRF major element concentrations were renormalized to 100% on an LOI-free basis.
Limits of detection (i.e. the smallest signal that can be quantitatively measures) are typically reported at the 6% confidence level (Potts et al. 1992); the limits of detection for major elements determined using fused beads manufactured from a range of basalt-derived alteration products are, in most instances, calculated as significantly less than 0.05 wt%. Conventionally, ensuring accuracy of XRF analyses is achieved by calibrating the instrument against a suitable range of reference materials. In this case, these reference materials included US Geological Survey basalt standards AGV-1, BCR-1 and BHVO-1, and also the laterite standards VL-1 and VL-2 (LaBrecque & Schorin 1987). After calibration, these basalt and laterite standards were then analysed as unknowns, and produced values typically <0.5% of the recommended values. The precision of XRF major element analysis is extremely high, and can be verified by replicate analyses (Potts et al. 1987). Replicate analyses (n >20) were performed upon the standard materials and demonstrate that error in reproducibility for major elements is typically less than 0.2 wt%.
Flame atomic absorption spectrometry
For the analysis of Mg, Ca, Na, P, Sr, Ba and Al, c. 3 g of powdered rock samples were subjected to digestion using aqua regia. Samples were digested for >16h before being transferred to a heating block, with further digestion at 80 [degrees]C for a further 3 h. Cooled distillates were filtered and 1 ml of 10% potassium chloride solution was added as an ionization suppressant. Duplicate blanks and test samples utilizing a standardized modern soil sample were also prepared for each batch analysed. Distillates were subsequently analysed in a Perkin Elmer flame atomic absorption spectrophotometer using air-acetylene and Nitrous oxide-acetylene to atomize the samples (Alien 1989). One sample was duplicated for every 20 analysed with an additional seven standards.
Flow injection analysis
For the analysis of N, P and K in rock samples, the samples were treated as soils, being crushed and subjected to Kjeldahl digestion using sulphuric acid. The sample and digestion reagent was heated in reaction tubes to 370 [degrees]C until clear and then filtered. Samples, including blanks and six standardized recent soil samples were then analysed using a Tecatot Enviroflow 5012 system comprising a 5027 sampler, 5012 analyser and a 5042 twin channel fixed wavelength detector. Analytical data were returned as values for NH^sub 4^-N and PO^sub 4^.
Palynology processing and analysis
Interbed samples were crushed to small fragments and the silicates dissolved in hydrofluoric acid until all grittiness was removed. The neutralized sample was then sieved using a 5 urn mesh nylon sieve, and the resultant residue treated with dilute nitric acid for <5 min to remove any pyrite. Neutralized residues were then pipetted onto glass coverslips using dilute PVA and mounted on glass slides as permanent strew mounts using a two-part epoxy medium. Slides were examined under a transmitted light microscope, with counts of up to 250 specimens being undertaken where the abundance of the palynomorphs allowed.
Nutrient flux within the Columbia River Large Igneous Province
Of paramount importance to weathering and biotic processes on lava fields is the climatic regime. It is fortunate that direct estimates of precipitation for the Columbia River area in the Miocene have been derived from palaeosol data from the Picture Gorge Subgroup (Sheldon 2003). This subgroup was erupted to the south of the main Columbia River Basalt Group lavas, and soils formed on flow tops within it give estimates of mean annual precipitation (MAP) between 600 and 1200mm. Similar high levels of precipitation along the Miocene west coast are also modelled by the general circulation model of Valdez et al. (1999).
The CIA-K transfer function (Maynard 1992), used to estimate the Picture Gorge Subgroup MAP by Sheldon (2003), was applied experimentally to XRF data from fluvio-lacustrine sediments from the Grande Ronde and Wanapum formations (Fig. 2). Although we are currently unable to correlate the values derived from this analysis directly to those from soil profiles, MAP values of between 1100 and 1400 mm are suggested. These are comparable with those derived from the model of Valdez et al. (1999), and are in the upper part of the range of the MAP values of Sheldon (2003) for the older Picture Gorge interbeds.
High levels of precipitation are an important factor in ensuring hydrological transfer of thermally fixed and volcanogenic N to the lava-field landscape. In dryer climatic zones, significant loss of NO^sub x^-rich ‘vog’ vectors nutrients away from the lava field to be dispersed in the wider environment, as happens on Hawai’i today (Heath & Heubert 1999). Similarly, high precipitation levels in the warm temperate Mid-Miocene west coast would have promoted rapid basaltic weathering. Samples taken from the Grande Ronde and Vantage Member interbeds sampled show higher CIA-K values than those at Locke Lake and Frenchman Springs Coulee, which give MAP values of between 800 and 1200 mm. Although this may represent a drying of the Mid-Miocene climate, the increasingly proximal location of the younger sites suggests that these values could reflect the mock aridity reported from other volcanic landscapes (Harris & Vancouvering 1995). However, these data make it clear that Columbia River Basalt Group weathering regimes and plant ecosystems were not precipitation limited, evidence further supported by the palynofloral record.
Fluvio-lacustrine sediments exposed at the Frenchman Springs Coulee section (Fig. 2) were deposited between the uppermost flow of the Frenchman Springs Member and the overlying Roza Member. These sediments yielded little in situ pollen, except for that of grasses; under the climatic conditions prevalent in the Mid-Miocene, this finding indicates that the vegetation had not progressed beyond an early successional stage (palynofloral data are available online at http://www.geolsoc.org.uk/SUP18310). However, the palynoflora also yielded abundant chlorophycean algae (Pediastrum bifidites), a taxon that occurs today in high frequencies in eutrophic water masses (Tappan 1980). Both flow injection analysis-FAAS and XRF analysis demonstrate that this Pediastrum bifidites abundance corresponds to evidence of raised macronutrient availability in both fluvial and lacustrine components of this system. It is possible that the high levels of PO^sub 4^-P^sub 2^O^sub 5^ recorded in these samples (see Supplementary Publication, above) were sourced from volatile volcanic sources and weathering of basaltic regolith (Fig. 3a and b). By comparing the major elemental composition (XRF data) of the Frenchman Springs Coulee interbeds with that of the underlying basalt, c. 100% residual base cations and P are recorded for the interbed profile. Comparison of these interbed residual data with residual P, Mg and Ca plots from Hawai’i interbeds (Vitousek & Farrington 1997; Chadwick et al. 2003) suggests that the Frenchman Springs Coulee interbeds were deposited over a period of <1 ka. Because of the mobility of NO^sub x^ in the post-depositional environment, evidence of N availability is preserved in the geological record only when N is fixed into carbonaceous material. Although this prevents the direct elucidation of NO^sub x^ levels, high levels of Mg-MgO suggest that macronutrient availability was elevated. Nitrate availability is itself attested to by the dominance of Pediastrum bifidites, which would have required high NO^sub x^ availability to attain such proportions in a fluvial or lacustrine system. In addition, high levels of P are predicted by the massive abundance of diatoms in laterally equivalent lacustrine sediments. Whereas our sample site is one dominated by fluvial processes, the laterally equivalent diatomites were deposited in lacustrine facies, which may have resulted from abnormally high P levels in an enclosed aquatic system. Here, the possibility that cyanobacteria contributed towards eutrophication cannot be ruled out, but is not supported by fossil evidence and is regarded as being unlikely given the oxygenated water mass of this lake system (Stewart 1977). Because of the short duration of this interbed, the low terrestrial biomass, the dominance of chlorophycean algae evident from the palynoflora and the proximity to an active fissure eruption system, we suggest that these data are consistent with the eutrophication of watercourses in the region by thermal fixation.
Sentinel Bluffs exposes the same interbed horizon as seen at Frenchman Springs Coulee, but in a more distal location (Fig. 2). Although algal cysts are common, and the sediments are highly diatomaceous, the algae do not attain the abundance levels seen at Frenchman Springs Coulee (see Supplementary Publication; see p. 960). At Sentinel Bluffs, the flora is dominated by the fern spore Laevigatosporites haardtii and angiosperm grain Nyssa sp. 1, suggesting an early to mid-successional vegetation (Collinson 2002), which is of moderate diversity. This increase in diversity reflects greater distance from the source of the eruptions and therefore, less environmental disturbance. In our analysis, a sample ordination plot (Fig. 4), shows a strong orientation of data points along axis 2, which exhibits a correlation (R^sup 2^ 50.3%) with our estimation of proximity to vent (Fig. 5). This regression is supported by a canonical correspondence analysis ordination plot (see Kovach 2002) for samples, which shows a linkage between increasing Al^sub 2^O^sub 3^ concentrations, reflecting increased weathering of drainage basin soil profiles, and the duration of the interbeds (Fig. 6).
Within the Columbia River Basalt Group, no other location contains evidence for algal dominance of the intensity seen at Frenchman Springs Coulee. This is in part attributable to the proximal to vent location of these sites, but also to their being the shortest duration interbeds examined (<1 ka). Further comparisons of residual %P with those in the Hawai'i data of Chadwick et al. (1999) suggest that other interbed horizons examined represent significantly longer durations of 10-100ka. Although the long duration interbed sections at Locke Lake and Palouse Falls both contained chlorophycean algae in the fluvial environmental system, there is no clear linkage between these minor influxes and nutrient availability.
Some important inferences can be drawn from these data. Our data demonstrate that macronutrients were available in abundance at the Frenchman Springs Coulee location, but that this is not apparent in sections >200 km from the volcanic source or of >103 years duration. The lithofacies of the Frenchman Springs Coulee and Sentinel Bluffs sections indicate deposition in rivers and ephemeral lake systems, and from the geochemical data it is apparent that these became a sink for macronutrients washed from the surrounding sparsely vegetated flow top terrain. The biota of the proximal location was concentrated into algal blooms, but the <1 ka duration of the interflow period did not allow establishment of vegetation communities beyond early successional stages.
Sites analysed from the Vantage Interbeds interval at Palouse Falls and Mill Creek Road are distal and far-distal, respectively, to eruption fissure locations, and show more complex sedimentary sequences with mid- to late successional plant communities sourcing Carya (wingnut), Cupuliferoipollenites (chestnut types), Inaperturopollenites (dawn redwood and taxodium), Tilia (lime) and Pityosporites (pines) pollen, in an association attributed to a conifer-hardwood, mixed mesophytic forest (Grey 1985). Pollen associations recovered from Palouse Falls and Mill Creek Road interbeds are correlated with evidence for low macronutrient availability. In particular, there is evidence of low levels of PO^sub 4^-P^sub 2^O^sub 5^ and Ca, with the lowest levels of Mg in the Palouse Falls section (Fig. 3). Indications of profile maturity support a mature sediment source for these fluvial sediments, with a low Ba:Sr ratio and high Al:base ratio (Fig. 7).
The distal location of the Mill Creek Road and Palouse Falls locations, some 300-450 km from the fissure vents, and the duration of the lengthy interflow period would have prevented any significant input from volcanically sourced N and P. The lowermost samples of the Palouse Falls site show common occurrences of Alnus pollen, indicating that this symbiotic N-fixer was an important component of the plant community. Such common occurrences of Alnus (alder types) may be attributable to early nutrient limitation (Fig. 8) with saturated soils (Thornton 2000), a correlation apparent in extant relatives of this plant today (Chapin et al. 1994; Hobbie et al. 1998).
A similar role for symbiotic N fixation by Alnus types is suggested by assemblages from within the lower units of the Douglas Canyon interbeds, Grande Ronde N2 group. Aspects of this palynoflora demonstrate that the vegetation community at this site did not progress beyond a mid-successional stage, illustrated by the association of abundant early successional polypodiaceous fern spores (Laevigatosporites haardtii; see Collinson 2002), with abundant Betulaceae. Comparison of percentage residual base cations and P in these samples with the Hawai’ian profiles of Chadwick et al. (1999) suggests an interbed duration of 4-8 ka. Both the XRF data and the composition of the palynoflora indicate that the early stage of interbed formation at both Paluse Falls and Douglas Canyon was a period in which macronutrients were of limited availability. Nutrient levels and the common occurrence of N-fixing species at these sites are comparable with those recorded in early to midsuccessional plant communities from contemporaneously inactive lava fields (e.g. Krakatau, Thornton 2000), where volcanogenic N is insignificant. Taking into consideration the distance of these sites from contemporary eruptive centres, and the 4-8 ka duration of these interbeds, it is suggested that these profiles characterize intervals in which volcanogenic NO^sub x^ did not play a contributory role in ecosystem development.
Our single site from within the Wanapum Basalt Formation at Locke Lake (Fig. 2) yielded a mid-successional palynoflora from a sequence of fluvial carbonaceous shales and sandstones. Frequencies of Laevigatosporites haardtii are high, but are accompanied by juglandaceous pollen including Caryapollenties veripites and Pterocarya sp., both more typical of mid-successional floras. Macronutrients are also recorded at intermediate levels, but it is noticeable that K, Na and Ca are raised, possibly explaining the lack of symbiotic N-fixers in this palynoflora.
The occurrence of volcanic ash within the Locke Lake succession provides evidence of an alternative vector for eutrophication. The abundance of K, Ca and Na with respect to Mg results from the input of felsic volcanic ash. At Locke Lake, the high K abundance (Fig. 9) is matched by peaks in Rb, indicating that excess K is derived not from metasomatism but from a volcanogenic source. Less exaggerated elevated levels of K and Rb are also detected in the Vantage Interbeds and Grande Ronde Formation N2 lavas sites, but the lower level of input did not appear to have a significant effect on the biota. The proximity of the Cascades Range to the Columbia River Basalt Group (Fig. 1) provides a nearby source of felsic tephra from eruptions, or by aeolian distribution of degraded ash. Similar records of high K levels were made by Sheldon (2003), in palaeosols of the older Picture Gorge Sub-Group. These palaeosols displayed K enrichment comparable with the younger examples reported here, suggesting that eruption of early Cascades Range volcanoes coincided with Columbia River Basalt Group activity over an extended period in the Mid-Miocene. Conclusions
Our data from the Columbia River LIP indicate that volcanically derived N and P may have made a significant contribution to ecosystem productivity in one interbed from the proximal zone of the lava field. Nutrients could have leached from the immature proximal soils into the lava-field fluvio-lacustrine drainage system, partly in response to the high rainfall, resulting in high algal and diatom productivity within the proximal zone. This may have been accentuated in fully aquatic lake systems where runoff would have concentrated macronutrients. One interpretation of our data suggests that elevated productivity in the proximal zone reduced or fixed significant proportions of volcanic N and P enrichment, there being no conclusive evidence of watercourse eutrophication in the middle or outer zones of the LIP. Biological fixation within the proximal zone would have resulted in the median to outer zones of the lava field developing plant communities adapted to low macronutrient availability, supplemented only by sporadic input of bases from acidic tephra erupted from the Cascades Range.
An alternative interpretation is supported by the lack of evidence for NO^sub x^ at the Palouse Falls location, which lies within the proximal zone. This interbed formed over an extended period (10^sup 3^-10^sup 4^ years) and displays low macronutrient concentrations throughout its duration. Here, we interpret these data to indicate that NO^sub x^ input was limited not only by distance from vent, but also by the coincidence of a contemporaneous eruptive phase. During interbed deposition, sporadic eruption, away from the sample site, would result in NO^sub x^ input without eruptive products mantling the sedimentary rocks. Without this contemporaneous eruption, eutrophication would not have occurred, creating a linkage between longer periods of eruptive quiescence, duration of interbed deposition and likelihood of volcanogenic eutrophication. Eutrophication by volcanogenic NO^sub x^ input would therefore have been most likely to occur during deposition of short- duration interbeds, which represent a transient break in eruption.
Where there is no eutrophication from volcanogenic NO^sub x^, evidence from the oldest interbeds within the medial to distal ranges of the Columbia River Basalt Group lava field signals a dichotomy in plant community succession. Species canonical correspondence analysis shows that two conifer-hardwood associations occur outside the zone of eutrophication from volcanically fixed P and NO^sub x^ (Fig. 8). One of these (N-deficient association, Fig. 8) is dominated by the N-fixing betulaceous taxa Betula and Alnipollenites verus, and represents the greatest diversity of the flora recorded. This association characterizes the successions at Douglas Canyon and Palouse Falls, both longer duration interbeds, calculated from residual base cations and P as 4-8 ka and 10-30 ka duration, respectively. These sites experienced little, if any, volcanic disturbance and were deposited over a greater time interval than more proximal sites, receiving a minimal nutrient supply from volcanogenic and weathering processes.
The second association (Fig. 8) is characterized by taxodiaceous pollen and the early succession forest marginal fern spore Laevigatosporites haardtii. Although this association is present across the whole range of medial to distal sites, it dominates the palynofloras from Locke Lake, which lack N-fixing species. This palynofloral composition is linked to the higher levels of input of Na, Ca and K from felsic Cascades Range sourced volcanic ash. Locke Lake is located well within the ashfall zone of these acidic volcanoes, and illustrates the short-circuiting of plant succession, bypassing the early N-fixer dominance of other interbed successions by extrinsic contribution of bases.
Although these data indicate that thermally fixed NO^sub x^ could have played a role in supporting lava-field biota within short- duration interbeds from the proximal zone of the Columbia River Basalt Group lava field, care must be taken in applying these conclusions to other large igneous provinces. Modem analogies highlight the role of climate and rainfall in lava-field ecosystems (Chadwick et al. 2003). Application of the CIA-K transfer function here and by Sheldon (2003) is important in this respect, establishing lava-field processes in the context of higher rainfall parameters. High MAP values and the interbed lithofacies of the Columbia River Basalt Group indicate that the lava field incorporated an active and perennial fluvio-lacustrine system. Where rainfall is seasonal or low, eutrophication vectors may be significantly modified in both geographical range and intensity of flux. Within the Columbia River LIP, contribution to macronutrient budgets from thermally fixed and volcanogenic NO^sub x^ has not been detected in the outer zone of the lava field, mitigating against this additional nutrient source playing an important role in ecosystems extrinsic to continental LIPs within similar climatic parameters.
This research was funded by NERC standard grant NER/A/2003/0444. The authors wish to thank two anonymous reviewers for their helpful comments.
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Received 20 December 2006; revised typescript accepted 6 February 2008.
Scientific editing by John Marshall
DAVID W. JOLLEY1, MIKE WIDDOWSON2 & STEPHEN SELF2
1 Department of Geology and Petroleum Geology, Meston Building, King’s College, Aberdeen AB24 3UE, UK
2 Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
Copyright Geological Society Publishing House Sep 2008
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