Paleoecological and Carbon Accumulation Dynamics of a Fen Peatland in the Hudson Bay Lowlands, Northern Ontario, from the Mid-Holocene to Present

Transcription

1 Paleoecological and Carbon Accumulation Dynamics of a Fen Peatland in the Hudson Bay Lowlands, Northern Ontario, from the Mid-Holocene to Present by Benjamin Cody O Reilly A thesis submitted in conformity with the requirements for the degree of Master of Science Department of Geography University of Toronto Copyright by Benjamin Cody O Reilly 2011

2 Paleoecological and Carbon Accumulation Dynamics of a Fen Peatland in the Hudson Bay Lowlands, Northern Ontario, from the Mid-Holocene to Present Abstract Benjamin O Reilly Master of Science Department of Geography University of Toronto 2011 Pollen assemblages, peat humification and carbon:nitrogen stratigraphy were examined at high resolution in a core from a fen peatland in the Hudson Bay Lowlands, Northern Ontario, to interpret the factors that drive long-term peatland dynamics. Subtle changes in the vegetation community are evident over the record, suggesting both allogenic and autogenic influences, but a fen community appears to have been resilient to external perturbations including isostatic rebound and hydroclimatic changes between 6400 and 100 years BP. Paleoclimatic reconstructions from the fossil pollen assemblages indicate that precipitation increased 3000 years BP at the end of the Holocene Thermal Maximum, and that carbon accumulation in the fen was controlled more by effective surface moisture (precipitation) than by temperature. The pollen record suggests changes over the past century, including increases in shrub Betula, Alnus, Ambrosia, and Cyperaceae and a decrease in Sphagnum spores, consistent with the observed Pan-Arctic shrub increase. ii

3 Acknowledgments I would like to start by thanking Dr Sarah Finkelstein for the guidance and assistance that she afforded me throughout my Masters. She was constantly keeping me motivated and excited about the next step, and was always so encouraging. I am very grateful for the opportunity that was given to me to work on this project. This project relied substantially on the funding and support of a number of sources. I wish to thank the Ontario Ministry of Natural Resources, the Ontario Ministry of Training, Colleges and Universities, the Natural Sciences and Engineering Research Council of Canada, the Wildlife Conservation Society of Canada and the Northern Scientific Training Program of the Department of Indian Affairs and Northern Development. Once the roads end, the cost of doing field research really takes off, and it couldn t be done without the generosity of these sources. The logistical support provided by Brian Steinback and the rest of the staff at DeBeers Victor Mine Environmental Lab is greatly appreciated. The stay at Victor was memorable, and I hope my torn pants were a lesson in proper field attire (or at the very least, a lesson in writing a proper near-miss card). I must admit, very few things cap off a day of walking around expansive muskeg like pulling a truck, so thanks for the staff at Victor for making us feel welcome! HMS PGB would never have sailed without the careful construction of Mircea Pilaf. Thank you for all your help over these two years Mircea! I also wish to thank Jim McLaughlin and Benoit Hamel for the core collection and supplemental site description. To the others in the Paleoecology Lab Carlos, John-Paul, Charlotte, Joan, Maara, Kristen and Nikki and those already moved on Jane and Jen, thanks for all the coffee breaks, patio beers, rants, discussions, assistance and good times. I owe you all a lot for the motivation iii

4 you afforded me, and for not laughing at my jokes resulting in me thinking of better ones! A special thanks to Kristen for helping sub-sample peat when the temperatures of the sediment lab approached solar-surface levels, and Joan for patiently sharing her vast knowledge of statistics with me. I would also like to thank Charlie and Jock for the visits, interesting conversations and helpful suggestions. I really need to thank my parents twice, mainly because I forgot to thank them in my undergraduate thesis acknowledgements, but more so because they encouraged me to take this opportunity and have been more supportive than I could have ever dreamed. I hope I can repay their kindness and goodwill! To the rest of the folks of PGB, thanks for making movie nights, Fridays, Chinese New Year and other events memorable. It really helped get through the tough parts of graduate school, and I will cherish this time forever. Lastly I d like to thank my girlfriend Tatiana for all her love and support during this time in my life. Thanks for all your encouragement and motivation, especially when I was at my grumpiest! I still think the Washington Redskins are better than the Philadelphia Eagles though! iv

5 Table of Contents TITLE PAGE i ABSTRACT ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS.v LIST OF TABLES viii LIST OF FIGURES ix LIST OF APPENDICES.xi CHAPTER 1: INTRODUCTION GENERAL INTRODUCTION AND OBJECTIVES Development of Northern Peatlands Rationale Proxies Utilized and their interpretation Ecosystem Resilience Peatlands as Complex Adaptive Systems LITERATURE REVIEW Holocene Climatic Transitions Past Paleoecological Studies Carbon Accumulation in Peatlands STUDY SITE...23 v

6 1.3.1 Study Region Site Description Climate of the study area Local and Regional Geologic Setting Quaternary Glacial History of the Hudson Bay Lowlands Post-glacial Isostatic Adjustment Local and Regional Vegetation..31 CHAPTER 2: METHODS FIELD METHODS LABORATORY METHODS..35 CHAPTER 3: RESULTS Pb DATING OF VICM_T3_SP AGE-DEPTH MODEL DEVELOPMENT PALEOECOLOGICAL RECONSTRUCTION BULK DENSITY C:N STRATIGRAPHY LORCA PEAT HUMIFICATION PALEOCLIMATIC RECONSTRUCTIONS..65 CHAPTER 4: DISCUSSION 76 vi

7 4.1 DRIVERS OF VEGETATION CHANGE CLIMATE RECONSTRUCTION CONTROLS ON CARBON ACCUMULATION DYNAMICS RESILIENCE OF THE VICTOR FEN ECOSYSTEM...92 CHAPTER 5: CONCLUSION CONCLUSIONS FROM THE VICTOR FEN RECORD FUTURE WORK.97 REFERENCES..99 APPENDIX A: RAW COUNTS OF VC vii

8 List of Tables Table 1: The three study proxies and their interpreted reconstruction..11 Table 2: Grain counts for the rationale of a 200 arboreal pollen grain count 40 Table 3: AMS radiocarbon dates for the Victor Mine Fen Core (VICM_T3_SP3)..48 Table 4: Percent carbon and nitrogen data used to test the homogeneity of the peat matrix.56 viii

9 List of Figures Figure 1: A map of past paleoecological studies in relation to the Victor fen..15 Figure 2: Postglacial emergence curves for the Victor fen site.31 Figure 3: The activity of 210 Pb in the uppermost Victor fen core section..45 Figure 4: Age-depth model derived for the Victor fen Core..47 Figure 5: Percentage pollen diagram from Victor fen core...52 Figure 6: Pollen Influx diagram for the Victor fen core 53 Figure 7: Bulk density of the Victor fen core 54 Figure 8: Percentage carbon in the peat sequence of the Victor fen core..57 Figure 9: Percentage nitrogen in the peat sequence of the Victor fen core...58 Figure 10: Carbon/Nitrogen ratio of the peat sequence of the Victor fen core.59 Figure 11: LORCA estimates for the entire peat sequence of the Victor fen core 60 Figure 12: LORCA estimates for the 60 cm to base section of the Victor fen core..61 Figure 13: LORCA estimates for the Victor fen core based on the age-depth model...61 Figure 14: Raw spectrophotometric absorbance results for the Victor fen core 64 Figure 15: Detrended absorbance values (A d ) for the Victor fen core...64 Figure 16: Reconstructed Average Annual Air Temperature for the Victor fen core...68 Figure 17: Reconstructed Average Annual Air Temperature of the most recent 2000 years for the Victor fen core...69 Figure 18: Reconstructed Average July Temperature for the Victor fen core..70 ix

10 Figure 19: Reconstructed Average July Temperature of the most recent 2000 years for the Victor fen core...71 Figure 20: Reconstructed Total Annual Precipitation for the Victor fen core...72 Figure 21: Reconstructed Total Annual Precipitation of the most recent 2000 years for the Victor fen core...73 Figure 22: Reconstructed Total June, July, August Average Precipitation for the Victor fen core.74 Figure 23: Reconstructed Total June, July, August Average Precipitation of the most recent 2000 years for the Victor fen core..75 x

11 List of Appendices Appendix A: VC01 raw pollen counts.112 xi

12 Chapter 1 INTRODUCTION 1.1 General Introduction and Objectives Peatlands store more carbon per unit area than any other terrestrial ecosystem (Dise 2009). However, most peatlands are located in the Boreal and Subarctic zones of the Northern Hemisphere, where the climate has been warming faster than anywhere else on Earth; this is a trend which is projected to continue (Meehl et al. 2007). An alarming consequence of global climate change is a decreased ability of certain ecosystems to uptake and store carbon. A decrease in carbon storage acts as a positive feedback to global climate, accelerating warming during periods of carbon release, and this warming is expected to impact peatland carbon cycling (Beaulieu-Audy et al. 2009). Highresolution paleo-data retrieved from peat repositories indicate that the carbon sink potential of northern peatlands has varied by an order of magnitude or more in past millennia (Yu 2006), in response to hydroclimatic change. Northern peatlands span an area of approximately km 2 and are thought to contain a carbon pool of between 270 to upwards of Gt of carbon (C), more than one third of the world s soil carbon (Beilman et al. 2009; Frolking et al. 2010; Gorham 1991; Turunen et al. 2002; Yu et al. 2010). For regions with permafrost (including continuous, discontinuous, sporadic and isolated zones), peat soils including Histels (perennially frozen peatland soils) and Histosols (unfrozen peatland soils) are estimated to contain Pg and Pg of soil organic carbon for North America and the total Northern Hemisphere respectively (Tarnocai et al. 2009). The wide variation in estimates of the total carbon pool supports the role of the paleo-record as an integral 1

13 2 element in quantifying future carbon storage capacity of northern peatlands under projected hydroclimatic conditions Development of Northern Peatlands The limited number of basal peat radiocarbon dates before circa years BP (based on 1516 basal radiocarbon dates of peat initiation from high latitude Europe, Asia and North America) suggests that there were none of the extensive peatland complexes (West Siberian Lowland, Hudson Bay Lowland (HBL)) that characterize the modern northern circumpolar region during the Last Glacial Maximum (MacDonald et al. 2006). This is supported by the near absence of Sphagnum spores in peat deposits from years BP (Gajewski et al. 2001). Sphagnum peatlands developed soon after deglaciation ( years BP) in North America, with initiation beginning in Alaska and the St. Lawrence regions, spreading eastward and westward respectively, in response to newly colonisable land (Gajewski et al. 2001; MacDonald et al. 2006). The arrival of early Holocene warming at years BP immediately following the Younger Dryas cold event is characterized by a rapid expansion of peatlands throughout the north (MacDonald et al. 2006). Carbon accumulation rates also peak at approximately 25 g C m -2 year -1 in the early Holocene (between and 9000 years BP, based on 33 northern peatland sites), concurrent with the peak in peatland initiation (MacDonald et al. 2006; Yu et al. 2010). However, Yu et al. (2010) use the previous northern peatland radiocarbon synthesis of MacDonald et al. (2006) with very poor spatial coverage of initiation dates in the HBL. The HBL represents an important gap in the complete spatial coverage of northern peatland

14 3 initiation (Gorham et al. 2007), especially because peatland initiation in the HBL took place after 7000 years BP. The North American initiation findings are consistent with rates from Alaska with the highest rate of peatland development (based on 284 basal peat dates) from to years BP (peak at years BP) (Jones and Yu 2010). The rate of additional peatland development in North America was constrained by the activity of ice retreat and land exposure. In boreal North America, major development occurred after 9000 BP, in response to the retreating Laurentide ice (Gajewski et al. 2001; MacDonald et al. 2006). The impact on today s atmosphere due to the establishment and growth of northern peatlands over the Holocene is that of a carbon sink (net deficit) of between GtC CO 2 (20-40 ppmv) and a source (net increase) of approximately 0.2 to 0.4 GtC CH 4 ( ppbv) as northern peatlands accumulate carbon (as the vegetation uptakes CO 2 ) and emit CH 4 through microbial production under anaerobic conditions (Frolking and Roulet 2007; Klinger et al. 1994). These northern peatlands have resulted in a radiative forcing cooling impact of -0.2 to -0.5 Wm -2. However, early in the Holocene the radiative forcing impact would have been a net warming of 0.1 Wm -2 (Frolking and Roulet 2007). The Hudson Bay Lowlands (HBL) of northern Ontario is the second largest peatland complex in the northern Hemisphere, after the West Siberian Lowland, and has been a significant contributor to the overall carbon pool that has accumulated in Northern peatlands during the post-glacial period (Beilman et al. 2009; Gorham 1991; Martini 2006). Due to the remoteness of the HBL, few stratigraphic reconstructions or carbon accumulation studies have been undertaken, pointing to a lack of understanding of the Holocene dynamics of this large peatland basin. The ability to accurately reconstruct past

15 4 environments is necessary to understand the dynamics of Earth systems, and to test models used to predict future hydroclimatological changes (Belyea 2007). Given the uncertain estimates of the total carbon pool in northern peatlands, and how these systems have responded to hydroclimatic change in the Holocene, high resolution analysis of vegetation change through pollen analysis coupled with estimates of carbon accumulation in the peat deposits of the HBL are crucial Rationale The objectives of this study were to reconstruct vegetation change and carbon storage in wetlands of the Attawapiskat River basin of the HBL and integrate these data sets with hydroclimatic changes inferred from paleoclimatic reconstructions. There remains considerable variability in the estimates of the carbon pool of northern peatlands. The variability in these estimates is due to possible inaccuracies in the basal dates of peat sequences as well as assumptions of average peat depth, average bulk density of peat and the proportion of carbon in peat (Gorham 1991; Turunen et al. 2002). Constraining these variables is important and this study aims to accurately characterize a poorly known region to refine estimates of the carbon pool, and how the pool has responded to climatic variability in the past. These objectives will be met by studying the paleoecological, paleohydrological and geochemical records retrievable from wetland sediments of the HBL. The records that were intensively studied include pollen assemblages isolated from the peat sediment, spectrophotometric humification of the peat matrix (the amount of humic acids at a given depth in the peat) and carbon:nitrogen ratios of the peat matrix. The ability of peatlands to accumulate autochthonous (originating at the site) material in

16 5 a sequential order, to sequester carbon as peat for many thousands of years, and to contain a very detailed record of changes in local to regional vegetation makes peatlands useful for investigating environmental and climate changes over Holocene or longer timescales (Chambers and Charman 2004). The autochthonous process of peat accumulation also makes peatlands less susceptible to redeposition, which is more common in lake sediment sequences and can impact stratigraphic results (Chambers and Charman 2004). The ultimate goal of this research was to characterize the effects of climatic (temperature and precipitation) and elevation (isostatic uplift) changes on vegetation communities and carbon accumulation in peat deposits of the HBL, and integrate these analyses with estimates of peat and carbon accumulation (Clymo 1984; Yu et al. 2003). Multiple paleoecological and paleohydrological techniques were employed to get a holistic picture of the history of climate change and carbon accumulation in the fen peatland. A multi-proxy approach is used to avoid erroneous interpretations from a single proxy, resulting in more robust reconstructions (Blundell and Barber 2005) Proxies Utilized and their Interpretation Pollen assemblages will be used to highlight and separate the influence of the allogenic (hydroclimatological) and autogenic (local biotic processes) factors on the carbon accumulation of each site. As ecotones, wetlands usually respond strongly to allogenic forcing (Mitsch and Gosselink 2007), highlighting the need to use proxies that can separate the biotic and abiotic drivers. Fossil pollen assemblages can be used to construct quantitative estimates of past environments using the modern analog technique (MAT) (Williams and Shuman 2008). The MAT is an established, robust procedure that assists

17 6 in the reconstruction of past climates and vegetation from quaternary fossil pollen assemblages when combined with modern, spatially extensive calibration datasets (Jackson and Williams 2004; Overpeck et al. 1985; Williams and Shuman 2008). The climate reconstructions from the pollen assemblages of the Victor fen will provide important paleoclimatic information for the study area, where it is lacking. The reconstructions chosen were average annual temperature ( C), mean July temperature ( C), total annual precipitation (mm), and average June, July, August (JJA) precipitation (mm). Each of these climatic values was chosen for a specific reason. Rates of Carbon sequestration in peatlands depend on the ambient hydroclimatic conditions (Belyea and Malmer 2004). The temperature values were chosen because the addition of carbon at the top of the acrotelm reflects the imbalance of fixation and aerobic decay; this relationship is affected by surface temperature of the peatland (Clymo et al. 1998). Precipitation values were chosen because past work has shown that carbon accumulation in fen peatlands responds strongly to even small changes in moisture conditions even if no change in dominant species is found in a paleoecological reconstruction (Yu et al. 2003). A summer subset of both temperature and precipitation was used because of the continental climate of the site. The winter period (being moist and cold at mid to high latitude) has been deemed less important to long-term surface wetness changes in mires (peatlands), with the exception of snow melt input in the spring possibly extending the season of surface saturation (Charman et al. 2009). Thus, precipitation reconstructed for the summer season was important. Also, given that humification values are surface humidity dependent and therefore, can exhibit a temperature or moisture signal, both reconstructions were necessary.

18 7 Studies on the mechanisms governing the vegetation dynamics of wetlands in the Holocene, through the analysis of pollen assemblages, have indicated that responses to climate-induced hydrological changes (allogenic) and within-wetland species change (autogenic) combine to facilitate succession (Singer et al. 1996; Winkler 1988). For example, moisture changes to the Portage Marsh basin (Indiana, USA), especially the transition from open-shallow lake to marsh, were coincident with changes in upland vegetation suggesting climate is the dominant mechanism driving the evolution from lake to marsh at that site (Singer et al. 1996). However, the progressive shallowing of the basin by the accumulation of autochthonous sediment has dampened to some extent the responses to climatic change, showing that both allogenic and autogenic influences determine wetland dynamics at this site (Singer et al. 1996). In Washburn and Hook Lake bogs of south-central Wisconsin, the major hydrological and aquatic vegetation changes were synchronous after 6500 years BP with the change to a dry-warm climate as shown through upland vegetation changes indicative of regional warming resulting in a lowering of the water table at both bog sites (Winkler 1988). A later transition to Sphagnum occurred at both sites, and the growth of established Sphagnum has been found to intensify the acidification process (Glaser et al. 1981) resulting in a greater influence of autogenic forcing. The synchronicity of changes points to climate being an important factor in influencing hydroseral change (sequence of ecological communities at a saturated site) in wetland ecosystems (Winkler 1988). The complex nature of the combination of allogenic and autogenic factors acting to force vegetation succession in peatlands necessitates proxies sensitive to both factors and this makes pollen analysis of the peat deposit useful.

19 8 Ratios of carbon:nitrogen will be combined with the pollen stratigraphy to accurately assign the various development stages (fen versus bog) in the cores, to assess the degree of decomposition and to calculate the carbon accumulation rate of each peatland development stage. The degree of peat decomposition (an analog for moisture) at each portion of the core is estimated through spectrophotometric measurement of peat humification. Measuring the absorbance of an alkaline extract of dried peat returns a result proportional to the amount of humic matter dissolved, with less absorbance indicating less humified peat (Aaby and Tauber 1975). A trend towards less humified peat (lighter coloured) suggests increasing mire surface humidity, which can be due to higher water table position and/or a more positive surface moisture balance, driven by either higher precipitation or lower temperature, or a combination of the two factors (Aaby 1976). At levels where subsamples for both humification and C:N ratios are possible, the correlation between the two variables will be assessed to determine how accurately the C:N ratios capture the decay signal. Given past work indicating that high N proportions and low C:N ratios are indicative of greater peat decay, the correlation is expected to be high (Belyea and Warner 1996; Borgmark and Schoning 2006; van der Linden and van Geel 2006). Table 1 is a summary of each proxy studied, the influencing factors acting on each proxy, and variables reconstructed by each proxy. As indicated by Loisel and Garneau (2010), the purpose of utilizing a suite of proxies is to attempt to isolate the mechanisms that drive peatland development, which include for the Hudson Bay Lowlands isostatic uplift, hydroclimatological variability and autogenic successional processes.

20 Ecosystem Resilience Past studies in the HBL have indicated that at many locations, a transition from a fentype ecosystem to a bog takes place over a long period of time. In light of this observation, the study of a long-lasting fen ecosystem provides a useful test of ecosystem resilience. Ecosystems are considered resilient when ecological interactions combine to strengthen one another and reduce disruptions (Peterson et al. 1998). This resilience denotes the maximum perturbation that can be absorbed by the ecosystem without causing it to shift to an alternate stable state (Scheffer et al. 2001). It has been defined as the capacity of a system to absorb a disturbance and reorganize while changing to retain the same structure, function, identity and feedbacks (Folke et al. 2004). The combination of proxies that capture vegetation and climate signals will aid in testing whether or not the fen is a true resilient ecosystem Peatlands as Complex Adaptive Systems Recently, peatlands have begun to be treated conceptually as complex adaptive systems (CAS) due to the important scale-transcending spatial and temporal linkages between the relatively fast near-surface processes and the slower processes occurring deeper in the deposits (Belyea and Baird 2006). The general properties of CAS that peatlands exhibit are spatial heterogeneity, localized flows, a self-organizing structure and non-linearity (Belyea and Baird 2006). The internal peatland dynamics and external forcing mechanisms both act to cause variability in hydroclimatological conditions and microrelief patterns, and the allogenic and autogenic forcings impact hydrological conditions influencing peatland carbon cycling and development (Belyea and Baird 2006).

21 10 The Victor fen exhibits characteristics of peatlands as complex adaptive systems as defined by Belyea and Baird (2006). The surface of the peatland being at or near the depth of the water table differs from the surrounding peatland micro-relief in hydrophysical and ecological characteristics. The water table being so close to the surface would influence the peat accumulation rate, the redox conditions in the acrotelm, and the local vegetation that could thrive under these conditions. This feature represents the spatial heterogeneity component of a complex adaptive system. The minerotrophic input represents the localized flow feature of a complex adaptive system, with the litter and peat layers interacting through the flow of water and nutrients (Belyea and Baird 2006). The size and shape of peatlands constrain processes operating at smaller scales. Thus, the peatland as a whole would influence the ecology and hydrology of the fen throughout its existence. This influence is referred to as the self-organizing structure component of complex adaptive systems (Belyea and Baird 2006). Lastly, the hydrological conditions at the surface of the peatland change with external forcing and varying minerotrophic inputs, and the changing conditions would constrain surface structure and composition as well as peat accumulation rates. This is the non-linearity component of a complex adaptive system (Belyea and Baird 2006). The consistent theme of a coupling of allogenic and autogenic influences acting on peatland dynamics provides further support for methods capable of separating the two dominant mechanisms. Establishing the relationship between hydroclimatic conditions and carbon dynamics is important because high LORCA (long term apparent rate of carbon accumulation, found by dividing the accumulated mass of C in a peat deposit by the age of the basal peat) (Korhola et al. 1995; Tolonen and Turunen 1996) values have

22 11 been found to be correlated both with wet or dry conditions (Loisel and Garneau 2010). Paleoecological analysis of pollen can isolate local and regional vegetation and climate changes (Chambers and Charman 2004) and together with peat humification, can be linked to carbon accumulation estimates through bulk density, age-depth model calculations and C:N ratios to determine the influence of hydroclimatic conditions on carbon dynamics in peatlands. Table 1: The three proxies utilized in this study, how they are isolated from the peat matrix, the signal they express, the external influences that force change in the signal and the interpreted reconstruction of each. Proxy Derived From Proxy Signal Controlled By Interpreted Reconstruction Pollen Organicwalled microfossils isolated from the peat matrix (1) Local-regional vegetation at/near study site (2) Regional pollen rain (1) Hydroclimatology (2) Isostatic Uplift (3) Air Masses Vegetation Reconstruction/ Succession Spectrophotometric Humification Humic acids chemically extracted from the dried peat matrix (1) Moisture content (2) Aerobic decomposition in the acrotelm (3) Water table depth (1) Peat forming vegetation (2) Differential resistance to decomposition (3) Compaction of peat Degree of peat decomposition and therefore, depth to water table C:N Ratios C:N bulk content of the dried peat matrix (1) Peat forming vegetation (2) Decomposition in catotelm (3) Peat accumulation (1) Hydrological inputs (ombrotrophy vs minerotrophy) (2) Residence time of peat in acrotelm Carbon accumulation estimates; Isolation of successional periods

23 Literature Review Holocene Climatic Transitions One of the key allogenic factors influencing peatland development is climate. Large scale climatic and hydrological changes during the period of peat deposition can strongly influence the rate that peat is deposited and sequestered (Zoltai and Vitt 1990). The Holocene Epoch is formally defined to have begun approximately cal year before 2000 AD, based on an abrupt shift in deuterium excess values, changes in δ 18 O and dust concentration changes, found within the NGRIP ice core from Greenland (Walker et al. 2008). The Holocene can be considered in three phases. The first phase coincides with the Boreal and Pre-Boreal zones, lasting from approximately to 9000 years BP, with insolation at a maximum at years BP due to the additive effect of the precession and obliquity orbital cycles but with some cooling effects from the remnant Laurentide Ice Sheet (Wanner et al. 2008). The second phase is known as the Holocene Thermal Maximum, Hypsithermal, Holocene Climatic Optimum or Atlantic zone and spans the period between approximately 9000 and years BP (Wanner et al. 2008). The Holocene Thermal Maximum was a period of continuing high summer insolation (lower than the year BP peak) in the Northern Hemisphere and a negligible climatic influence of the Laurentide Ice Sheet on a hemispheric scale (Wanner et al. 2008). The Holocene Thermal Maximum began approximately years BP in the westernmost regions of Arctic and Subarctic North America. However, its onset was delayed in the Hudson Bay Lowlands (perhaps spanning years BP, McAndrews et al. 1982; McAndrews and Campbell 1993) as the remnant Laurentide Ice

24 Sheet had kept the proximal region cool through its impact on the surface energy balance (Ritchie et al. 1983; Kaufman et al. 2004). 13 The third phase is the Subboreal and Subatlantic zones lasting from the terminal Hypsithermal to the Pre-industrial (100 years BP), and is commonly referred to as the Neoglacial period during which summer insolation declined in the Northern Hemisphere (Wanner et al. 2008). The conditions during these major subdivisions of the Holocene influenced the establishment of northern peatlands as well as their expansion and succession throughout the Holocene (MacDonald et al. 2006). The terrestrial system is sensitive to these changes in global insolation but the climate response to this forcing is dependent on the amount of radiation, its seasonal distribution across the planet and feedback mechanisms (including ice cover, albedo, ocean and atmospheric circulation) (Beer et al. 2000). While the HBL is underreported in relation to the coverage of peat basal dates (Gorham et al. 2007; MacDonald et al. 2006; Yu et al. 2010) and carbon accumulation estimates, studies have been undertaken on the evolution of the landscape during the Holocene by focusing on stratigraphic studies of peat profiles. These pioneer works will be discussed to indicate what is known about the HBL, and to illustrate knowledge gaps in the paleoecology of the region, providing a further rationale for this thesis Past Paleoecological Studies Previous paleoecological studies from the HBL (see Fig. 1 for locations) provide some insight into the relative importance of allogenic and autogenic processes in determining peatland vegetation changes. Terasmae and Hughes (1960) developed a pollen diagram

25 14 for a section along the Attawapiskat River, approximately 90 km west-northwest from the fen study site (Fig. 1), and it is the closest paleoecological reconstruction available for comparison. The section is approximately 150 cm in length, with 100 cm of Sphagnum peat overlying 30 cm of strongly decomposed woody-fen peat which in turn grades into peat with clay, brown clay and finally marine clay (Terasmae and Hughes 1960). The clayey peat contains foraminifera, indicative of brackish water at the site, and the high proportion of Cyperaceae pollen signifies a salt marsh (McAndrews et al. 1982; Sjors 1963; Terasmae and Hughes 1960). The basal peat was dated to 5430 ± 160 cal year BP at a depth of approximately 131 cm (Sjors 1963; Teramae and Hughes 1960) (all dates henceforth are expressed as calibrated years before present; if dates were not calibrated by original authors, they were calibrated using the program CALIB (ver 6.0.1) and the INTCAL09 calibration curve) (Reimer et al. 2009; Stuiver and Reimer 1993). The peat section begins as a fen, followed by a period of bog development (Terasmae and Hughes 1960). The authors do not make any climatic inferences from the diagram and instead focus on a successional change from fen to bog. However, given the single basal date to develop a rough chronology, the peaks in Sphagnum spores between approximately 4480 to 1950 years BP may correspond to the high proportions of Sphagnum found by other authors, beginning between 3400 and 2500 years BP, and interpreted as evidence of Neoglacial cooling (Kettles et al. 2000; Klinger and Short 1996; McAndrews et al. 1982). Sjors (1963) proposed (based on the diagram by Terasmae and Hughes 1960) that the landscape evolved from a brief intertidal salt marsh to a swamp forest and then to a woody fen and finally developed into a bog, following a direction of increasing wetness and a decrease in minerotrophic inputs.

26 15 Figure 1: A map of the locations discussed in this section together with the location of the Victor fen site of this study and the surrounding communities of the Hudson Bay Lowlands. This pioneer record needed to be improved upon because it was relatively short, coarsely dated and did not capture the important influence that climate has on peatland evolution. McAndrews et al. (1982) developed a pollen and macrofossil diagram from R Lake (approximately 180 km north of the fen site, Fig. 1) in order to develop a longer record. An estimate of lake emergence from the former Tyrrell Sea based on a chronology developed from two radiocarbon dates and the modern sediment surface (extrapolated to the basal depth) was years BP. The pollen record indicated that there was a succession from sparse coastal tundra, dominated by Dryas, willow, sedges and grasses, to shrub tundra, dominated by shrub birch (Betula pumila), to the modern woodland between and years ago, in response to the decreasing influence of

27 16 the retreating Tyrrell Sea (McAndrews et al. 1982). The presence of forbs including Najas flexilis (seed macrofossils) between 6500 and 3000 years BP was interpreted as evidence for the Holocene Climatic Optimum, recorded contemporaneously on the eastern shores of Hudson Bay (Gajewski et al. 1993; Kaufman et al. 2004). However since 2500 years BP, the macrofossil record indicates a decrease in tree abundances, and both Sphagnum bogs and bog forests have become more dominant suggesting some evidence for Neoglacial cooling and heightened rates of paludification (McAndrews et al. 1982). Paludification is defined as the process of bog expansion caused by a gradual rise in water table as the accumulation of peat impedes drainage (National Wetlands Working Group 1988). While the record of McAndrews et al. (1982) shows a strong role for climate in driving vegetation change, Klinger and Short (1996) found that hydrological changes driven by isostatic rebound and autogenic processes were important at the Kinosheo Lake bog site in the southern HBL (Fig. 1). Regional pathways for vegetation change over time were proposed based on land cover types and abundances from Landsat imagery, aerial photographs and ground vegetation surveys. These land cover types were used to reconstruct successional pathways through time as changes may be inferred from the sequences identified of different age communities in a spatial array (Klinger and Short 1996). Thus, these successional pathways are based on substituting distance from the present coast for time before present. The moist site pathway represented mesosere (sequence of ecological communities with a balanced moisture supply) primary succession in the more low-lying, extensive areas between beach ridges, leading to a black spruce bog forest in approximately 2000 years of landscape evolution.

28 17 The paleoecological reconstruction of a peak block profile also from the region of Kinosheo Bog (basal date of 4110 ± 80 years BP) identifies three distinct periods: an early succession zone high in herbs, Pinus, and Picea lasting between 500 and 1000 years, a period of maximum development of Picea woodland and a increase of Sphagnum between 3400 to 2500 years BP, and finally a period of Sphagnum dominated peatland with abundant ericaceous shrubs and an increase in ferns from 2500 years BP to the present. The pollen influx patterns at the site were found to be very similar to expectations from patterns derived from the regional moist-site chronosequence (Klinger and Short 1996). The authors proposed that the mechanisms driving landscape development in the Hudson Bay Lowlands involve a coupling of succession, hydrology, topography and climate (Klinger and Short 1996). As succession takes place over a significant period of time, the factors of topography and physically controlled groundwater hydrology seem to become less important than biotic (autogenic) and climatic influences. Subsequent work at the Kinosheo Lake bog determined that large scale Holocene climate variations had a greater role than isostatic rebound in the evolution of that peatland. Kettles et al. (2000) analyzed microfossil, macrofossil and geochemical stratigraphy in a peat core from Kinosheo Lake bog (Fig. 1). It was proposed that this bog formed by paludification processes as no evidence of an aquatic fen stage was found in the early peatland record (basal section dated to 4000 ± 80 years BP). A similar succession was put forth by Klinger and Short (1996) for the same peatland. In the period of 4000 to 2500 years BP, pollen assemblage diversity declines in response to the establishment of a Sphagnum-dominated peatland due to cooler conditions (Kettles et al.

29 ). The subsequent decline in Picea pollen during the last 2500 to 2000 years was indicative of more open forest cover, and combined with the increased amount of Sphagnum spores supports a regression of forest cover consistent with the cooling trend that was observed further east (Gajewski et al. 1993, Kettles et al. 2000). This finding is consistent with the increased proportion of Sphagnum found by McAndrews et al. (1982). Kettles et al. (2000) contended that the major changes in the record are a function of Holocene climate changes even if (as indicated by peat geochemical data) ecological succession(s) over time also shape peatland dynamics. This supports the common theme of allogenic and autogenic factors both influencing long term dynamics in vegetation change in peatlands. Using multiple peatlands along a regional chronosequence of isostatic rebound (akin to the chronosequence studied by Klinger and Short (1996)), Glaser et al. (2004a) sought to corroborate the importance of isostatic rebound on peatland evolution. Glaser et al. (2004a) investigated the stratigraphy of three raised bogs in the Albany River basin (Fig. 1) along the regional chronosequence, which is reflected in the age of the sites from the youngest (Belec Lake Bog) nearest the coast to the oldest (Oldman Bog) furthest from the coast (Glaser et al. 2004a). The depth of the peat profile also increases inland from the shallowest at Belec Lake to the deepest at Oldman (Glaser et al. 2004a). Analyses of pollen, plant macrofossils and carbon:nitrogen ratios of the peat deposit were all utilized to investigate the dynamics of bog development. The three bogs exhibit similar pollen stratigraphies (and the same stratigraphic units), with four distinct zones representing the succession of vegetation at each site. The basal zone is interpreted as a tidal marsh at all three sites. This zone is overlain by a fen forest followed by a bog

30 19 forest, the change supported by an increase in the carbon: nitrogen ratios between zones two and three. This succession is explained by the nitrogen-deficient nature of bog ecosystems, a condition prevalent until other nutrients become limiting (Kuhry and Vitt 1996). The final zone is interpreted as a non-forested bog. This succession from marsh to fen to bog at all three sites mimics that found by Terasmae and Hughes (1960). This shared stratigraphy between the different bogs suggests that the peatland succession followed the same pathway at each site, driven by geological processes, primarily the isostatic rebound of the region. The authors concluded that the differential pattern of uplift, which reduces the regional gradient and raises water table levels, is the primary driving factor of peatland genesis in the Hudson Bay Lowlands. The bog development conformed to a simple predicted pathway indicating a conservative response of the local biota to the regional environment (Glaser et al. 2004a; Glaser and Janssens 1986) but the influence of long-term variations in hydroclimatology (especially the climatic conditions during the major subdivisions of the Holocene) was ignored. More recently, Loisel and Garneau (2010) investigated two peat bogs (Lac Le Caron and Mosaik, Fig. 1) in the James Bay Lowlands of Northern Quebec using a multiproxy approach (involving the analyses of plant macrofossils, testate amoebae, peat humification, bulk density and C:N ratios) in order to assess whether hydroclimatic changes resulted from autogenic or allogenic factors. The plant macrofossil based reconstructions provided a more robust understanding of peatland dynamics (than just the inferences made from the testate amoebae) through identifying the patterns of vegetation succession at the sites. However, the testate amoebae captured short-term (multi-decadal)

31 hydrological changes and were more sensitive indicators of moisture conditions than the macrofossils. 20 Two synchronous changes in hydroclimatology were isolated between the two peatlands with humid conditions around 1000 years BP and wetter conditions from 250 years BP to the present, interpreted by the authors as indicative of the Medieval Climate Anomaly and the Little Ice Age respectively (Loisel and Garneau 2010). These two large climatic anomalies were not identified or described by the authors of the other studies; the resolution of this study is higher than that of Glaser et al. (2004a), Kettles et al. (2000) and McAndrews et al. (1982), which may explain why it was able to capture shorter-term climatic changes. The synchronicity between the two sites indicates regional allogenic forcing on peatland development. Site-specific autogenic forcing was also identified through the differences between the cores taken from the ribbed sections of the peatlands and from those taken from the pool sections reflecting the local geomorphic and hydrological states (Loisel and Garneau 2010). The isolation of the relative contribution of both allogenic and autogenic influences on peatland dynamics reaffirms the importance of combining multiple proxies to separate potential drivers whenever possible (Blundell and Barber 2005) Carbon Accumulation in Peatlands Each successional study that has been conducted indicates that the HBL often evolves towards a Sphagnum dominated peatland. In this and other types of peatlands, each year s cohort of litter undergoes some aerobic decay and is buried under the weight of younger material, until the main plant structure collapses. Eventually, the organic material becomes waterlogged and anaerobic, where decay happens a thousand times

32 21 slower than near the surface, thereby sequestering carbon on long time scales (Belyea and Clymo 2001). Most peat-forming ecosystems consist of two layers (and are referred to as diplotelmic): the upper acrotelm, an aerobic layer of high hydraulic conductivity where decay is relatively high, and the lower, thicker catotelm, an anaerobic layer with lower hydraulic conductivity and much lower rates of decay (Clymo 1984; Ingram 1978). The boundary between the two layers corresponds to the mean depth of the minimum water table in the peat profile during the summer (Clymo 1984). The above- and below-ground components of plants (litter) growing on the surface of the peatland decomposes rapidly in the acrotelm, due to such processes as the leaching of soluble organics (Belyea and Malmer 2004; Yu et al. 2001). During passage through the acrotelm, the peat becomes progressively more enriched in the more slowly decaying components, or recalcitrant components, and selective decay may continue in the catotelm, under anaerobic conditions. Thus, the specific composition of peat at depth becomes an increasingly inaccurate representation of the surface vegetation that formed the deposit (Clymo 1984). Litter decay is most rapid in the zone of water table fluctuation, least in waterlogged peat, and intermediate in the oxic acrotelm above the water table (Belyea and Clymo 2001; Ingram 1978). The decaying plant material transitions to peat and is submerged at the base of the acrotelm by the rising catotelm/water table and becomes anoxic as the consumption of molecular O 2 by microbial life forms exceeds the rate at which O 2 can diffuse down through the water from the air (Clymo et al. 1998). As peatlands can be very long lasting ecosystems, a long-term rate in organic carbon accumulation (LORCA) becomes a meaningful measure

33 22 to quantify how this sequestration mechanism is influenced by internal and external forcings. Throughout the Holocene, estimates of the average LORCA for northern peatlands range between 16.2 g C m -2 year g C m -2 year -1, and 44.1 g C m -2 year -1 (Beilman et al. 2009; Gorham 1991; Yu et al. 2010). However, some studies have found higher LORCA estimates for certain peatland types (fens and marshes) of upwards of g C m -2 year -1 (Botch et al. 1995). Similar factors that produce uncertainty in the total carbon pool estimates, including average peat depth, average bulk density of peat and the proportion of carbon in peat combine to result in uncertainty in LORCA measurements (Botch et al. 1995; Gorham 1991; Turunen et al. 2002). Bogs typically have a higher LORCA and accumulation is more uniform and predictable than fens, and accumulation tends to decrease from the more southerly peatlands (boreal) to the more northerly (Subarctic) (Beilman et al. 2009; Tolonen and Turunen 1996; Turunen et al. 2002; Zoltai 1991). Past data sets have contributed to the range of uncertainty surrounding LORCA values because they are biased in the inclusion of profiles almost exclusively from the centre of mires, where peat was the deepest (thus under-representing shallow mires), and from terrestrialized basins at the expense of paludified mires (Turunen et al. 2002). Carbon accumulation also tends to be more rapid at younger mires as opposed to older mires, with a clear increase in LORCA for peat columns younger than 5000 years (Tolonen and Turunen 1996). The generally cool, moist climate during the Holocene has tended to favour C accumulation and maintained the boreal and Subarctic sink of carbon in peatlands (Turunen et al. 2002). In Canada, the major period of peat (and therefore carbon)

34 23 accumulation at the northern border of the boreal forest was the early to middle Holocene, when summers were warmer than present (Ovenden 1990). Mid post-glacial climates were unfavourable for peat growth except in northern peatlands, while the accumulation rates have become lower towards the present (Sjörs 1980). LORCA is influenced by decay (the actual rate of carbon accumulation is lower due to some amount of plant decay in the anoxic zone of the peat), but LORCA still provides useful insight into the dynamics of carbon input and decay (Clymo et al. 1998; Korhola et al. 1995; Turunen et al. 2002). The humification analysis of the Victor fen core was included to try to account for the influence of decay. A subsequent study of the same two peatlands studied by Loisel and Garneau (2010) determined that their Holocene C accumulation rate was 18.9 and 14.4 g C m -2 year -1, for Lac Le Caron and Mosaik respectively (Van Bellen et al. 2011). The late Holocene reduction in long term C accumulation at these sites (which was a continuation of a gradual slow down) was attributed to both autogenic (local water table mound conditions) and allogenic (climate change) factors (Van Bellen et al. 2011). A new estimate of LORCA for the fen peatland studied is another objective of this research. 1.3 Study Site Study Region Peatlands cover approximately 12% of the present land area of Canada, with 97% of these peatlands occurring in the boreal and subarctic wetland regions (Tarnocai 2006), two ecoclimatic regions dominated in Ontario by the nearly unbroken extensive peatland basin of the Hudson Bay Lowland (Sjörs 1963). Extensive peat basins are unique regions

35 24 where the factors of climate, landscape and local biota produce high water tables that facilitate the expansion of peatlands into adjacent areas (Glaser et al. 2004b). More than 90% of the Lowland itself is a saturated peatland ecosystem, and these organic deposits range from 0.5 m to upwards of 4 to 6 m deep (Martini 2006; Pala and Weischet 1982; Riley 2003). The depth of peat accumulation is a function of the length of time that the site has been exposed above water, the topography of the underlying material (glacial till or marine sediment) and the distance of the site from the present coastline of Hudson- James Bay (Glaser et al. 2004a; Pala and Weischet 1982; Martini 2006). There appears to be a strong correlation of peat depth, elevation and distance from the coastline below 65 m a.s.l. for open and treed fens in the High boreal wetland region (Riley 1982) Site Description The immediate area of the study site is dominated by the near complete coverage of peatlands (~90%) (Tarnocai et al. 2000). This peatland cover comprises 55% bogs, and 35% fens (Tarnocai et al. 2000). Bogs are distinguished by having a water table at or near the surface, with the surface virtually unaffected by nutrient rich groundwater (and are therefore low-nutrient ecosystems) whereas the fens have a water table at or just above the surface with waters rich in nutrients originating from mineral soils, and a very slow internal drainage by seepage down low gradient slopes (Zoltai 1988). The dominant vegetation of the site was categorized according to the Canadian Forest Ecosystem Classification. There was no coverage of trees taller than 10 m. The trees or shrubs between 2 and 10 m high were represented by Larix laricina with coverage of 40%. The trees or shrubs m and <0.5 m height categories contained Betula pumila with coverage of 50% for both. There was herbaceous cover between 75 and 100% comprised