Browsing by Author "Baker, Paul A"

This dissertation investigates the limnology of the southwestern Peruvian Amazon centered on the Madre de Dios department by examining first the geomorphology and then the ecology and biogeochemistry of the region's fluvial systems.

Madre de Dios, Peru is world renowned for its prolific biodiversity and its location within the Andes biodiversity hotspot. It is also a site of study regarding the development of the Fitzcarrald Arch and that feature's geomorphological importance as the drainage center for the headwaters of the Madeira River - the Amazon's largest tributary and as well as its role as a physical divider of genetic evolution in the Amazon. Though each of these has been studied by a variety of prominent researchers, the ability to investigate all the aspects of this unique region is hampered by the lack of a regional geomorphological map. This study aims to fill that gap by using remote sensing techniques on digital elevation models, satellite imagery and soil, geology and geoecological maps already in publication to create a geomorphological map. The resulting map contains ten distinct landform types that exemplify the dominance of fluvial processes in shaping this landscape. The river terraces of the Madre de Dios River are delineated in their entirety as well as the various dissected relief units and previously undefined units. The demarcation of the boundaries of these geomorphic units will provide invaluable assistance to the selection of field sites by future researchers as well as insights into the origin of the high biodiversity indices of this region and aid in planning for biodiversity conservation.

Secondly this study examines 25 tropical floodplain lakes along 300 km of the Manu River within the Manu National Park in the Madre de Dios department. Alternative stable state and regime shifts in shallow lakes typically have been examined in lakes in temperate and boreal regions and within anthropogenically disturbed basins but have rarely been studied in tropical or in undisturbed regions. In contrast this study focuses on a tropical region of virtually no human disturbance and evaluates the effects of hydrological variability on ecosystem structure and dynamics. Using satellite imagery a 23 yr timeline of ecological regime shifts in Amazon oxbow lakes or "cochas" is reconstructed. The study shows that almost 25% of the river's floodplain lakes experience periodic abrupt vegetative changes with an average 3.4% existing in an alternative stable state in any given year. State changes typically occur from a stable phytoplankton-dominated state to a short lived, <3 yr, floating macrophytic state and often occur independent of regional flooding. We theorize that multiple dynamics, both internal and external, drive vegetative regime shifts in the Manu but insufficient data yet exists in this remote region to identify the key processes.

To complete the investigation of tropical limnology the third study compares and contrasts the nutrient-productivity ration of floodplain and non-floodplain lakes globally and regionally. For over 70 years a strong positive relationship between sestonic chlorophyll-a (Chl-a) and total phosphorus (TP) has been established with phosphorus generally viewed as the most limiting factor to productivity. Most of these studies, however, have focused on northern, temperate regions where the lakes are typically postglacial, isolated and fed by small streams. Relatively little work has been done on floodplain lakes which are semi or permanently connected to the river. This study examines the relationship between nutrients and productivity in floodplain lakes globally through an extensive literature synthesis. Values for total phosphorus, total nitrogen and chlorophyll-a were collected for 523 floodplain lakes, represented by 288 data points while 551 data points were collected for 5444 non-floodplain lakes. Analysis revealed that globally, floodplain lakes do not show any significant difference in the total phosphorus/chlorophyll-a relationship from that found in non-floodplain lakes but significant differences are seen between tropical and temperate lakes. We propose that the term `floodplain' lake should serve as purely a geographical descriptor and that it is lacking as an ecological indicator. Instead factors such as precipitation seasonality, hydrological connectivity and regional flooding regimes are better indicators of high or low productivity in floodplain lakes.

The Peruvian/Bolivian Altiplano is an important hydrologic system for paleoclimate reconstruction because it is unique in its ability to record climate variability associated with the near-continental scale South American summer monsoon (SASM), which is responsible for much of the precipitation over the Amazon basin and the southern subtropics. Over long timescales moisture on the Altiplano fluctuates in intensity due to changes in precessional insolation forcing as well as teleconnections to decadal-to-millennial scale abrupt temperature shifts in the Northern hemisphere Atlantic. These long-term changes in moisture transport to the Altiplano have been observed in multiple paleoclimate records, including drill core records and paleo-lake level records, as apparent advances and retreats of large lakes in the terminal basin occupied by the Salar de Uyuni and the Salar de Coipasa.

Presented here are the results from three studies that utilize different methods to create a refined reconstruction of paleohydrology, as well as paleoclimate, on the Altiplano. A major goal of this research is a more detailed understanding of millennial scale climate variability as it relates to insolation changes and abrupt warming and cooling in the north Atlantic. The first study discusses the creation of a paleohydrologic profile to reconstruct north-south hydrological history using previously reported lake core sediment records the northern and southern basins of the Altiplano, and a new 14 m core from the Salar de Coipasa representing the last ~45 ka. The second study uses a terrestrial hydrology model to simulate lake level changes through time given changes in precipitation and temperature. The third study uses strontium isotopic measurements of carbonates and halites in a 220-m core from the Salar de Uyuni to determine how source waters to the southern basin have changed through time.

The paleohydrologic profile in the first study is constructed using records from three major basins within the Altiplano: Lake Titicaca in the north, and Salar de Coipasa and Salar de Uyuni in the south. The new continuous sediment core from Salar de Coipasa indicates a lake that has fluctuated between deep and shallow phases for the last 45 ka. Lacking sufficient calcium carbonate, we instead take advantage of the general correlation between d18O and d13C in closed basin lakes to approximate water balance using d13C from organic carbon. This reconstruction is validated with diatom paleoecological records. The isotope measurements and diatom records indicate that from 45-36 ka Coipasa was moderately deep, consistent with paleoshoreline evidence of paleolake Minchin (46-36 ka). From 36-26 ka a shallow lake <10 m deep occupied the Coipasa basin. During the LGM (26-21 ka) the lake varied from moderate to shallow and during the Holocene (< 10 ka) the lake evolved from a shallow lake to a salt flat.

The hydrologic model in the second study was run through many scenarios including increases in precipitation, decreases in temperature, and combinations of the two. During the LGM southern Altiplano lakes fluctuated between 3,660 - 3,700 masl. Model results suggest that during this period basin wide precipitation increased up to 250 mm/yr compared to modern values dependent on a temperature decrease of 5 °C relative to modern values. To create a lake at elevation 3,760 masl consistent with the highest paleolake phase (Tauca, ~16 ka) the model requires an increase of 350 mm/yr compared to modern values dependent on a 5 °C decrease in temperature (relative to modern values). An increase in temperature alone of 2 °C above modern values causes Lake Titicaca water level to decrease ~30 m, creating a closed basin lake. Results indicate that Lake Titicaca outflow is necessary to sustain large lakes in the southern basin, providing ~40-60% of total input via the Rio Desaguadero.

Analysis of a 220 m core from the Salar de Uyuni suggests periods of alternating wet and dry phases (indicated by alternating mud and salt units respectively) at the salar. Evident in the record is a transition at ~60 ka from sediments consistent with dry conditions ("playa lakes") to sediments consistent with deep lakes ("great lakes"). It has been shown that rivers and lakes in the Bolivian and Peruvian Altiplano display a range of Sr isotopic ratios that can be connected to the lithologies of specific drainage basins. Measurements of Sr ratios of the alternating halites and carbonate sediments are used to determine when paleolakes in the Salar were supplied by flow from the northern and central basins of the Altiplano, and when they were more a product of increased precipitation in the Uyuni basin. The results from Sr isotope analysis suggest that prior to ~60 ka the primary source of Sr to the Uyuni was local runoff and direct precipitation. Following the state change from the "play lakes" phase to the "great lakes" phase Sr isotope measurements suggest a significant influence from more radiogenic waters originating in the central and northern Altiplano basins. The reason for this state change is attributed to a combination of a general increase in precipitation following the onset of the MIS-4 (~70 ka) glacial period and downcutting of the Laka Jahuira hydrologic divide, which connects Lago Poopó in the central basin to the Salar de Coipasa.

This approach of reconstructing hydrology using the combination of multiple paleolake records, hydrological modeling, and isotopic tracers allows for a better understanding of how precipitation and temperature changes affect the advance and retreat of large lakes on the Altiplano, and ultimately a more accurate understanding of how decadal-to-millennial forcings influence the climate of the subtropical Andes.

The Amazon basin is a center of deep atmospheric convection and thus acts as a major engine for global atmospheric circulation. From this basin, one fifth of the world's freshwater flux is discharged into the Atlantic and nearly two-thirds of the global rain forest resides herein. Yet despite its significance, little is known about past Amazon climate variability and the response of the forest ecosystem to climate.

Here, I attempt to reconstruct the paleoclimate history of a portion of the Amazon basin using both tree ring growth and the carbon and oxygen isotopes of tree ring cellulose from the Madre de Dios department of Southeastern Peru. Bomb 14C dating identifies annual rings in tropical species Cedrela odorata and Dipteryx micrantha. A ring width chronology spanning 189 years (1817-2006) is developed for Cedrela odorata and 5 trees of Dipteryx micrantha are utilized for isotope reconstruction. The oldest tree used in the isotope reconstructions has an error-adjusted age of about 473 years (1533-2006). Using the species Cedrela odorata, Ceiba pentandra, Hymenaea courbaril, Myroxylon balsamum, and Tabebuia serratifolia, I develop 5 tree ring chronologies using relative ring width measurements and chart the growth behaviors of over 40 trees. Ring width chronologies from tree species Cedrela odorata and Ceiba pentandra show a significant correlation with wet season precipitation (r = 0.42, and 0.37, respectively, p<0.05). The ring width chronology developed from the species Hymenaea courbaril is significantly correlated (r = 0.68, p<0.05) with January river discharge. Correlations between wet season precipitation and ring growth in Cedrela odorata are used to identify extreme wet and dry events. Nine historic droughts of the 20th century are identified in the C. odorata record. An increase in the frequency of extreme events (mean recurrence interval = 5-6 years) is observed in the 20th century and both Atlantic and Pacific sea surface temperature (SST) forcing mechanisms are implicated. The chronology shows a moderate correlation with both ENSO and tropical North Atlantic SST anomalies, suggesting that both basins play a role in precipitation variability over tropical South America.

Carbon and oxygen isotopic measurements (proxies of moisture stress and precipitation amount, respectively), in tree ring cellulose from 5 Dipteryx micrantha trees are used to reconstruct an error adjusted 473 year long record of precipitation variability. Because an error correction factor is applied to the chronologies of the trees of this species, assessment of annual-scale variability is precluded. Instead lower frequency trends are examined. No long term trends are identified in the oxygen isotopic records from individual trees. The carbon isotopic records of all five individual trees track the depletion of atmospheric δ13C during the 20th century due to the anthropogenic input of fossil fuel CO₂ (The Suess effect). Relatively large variability in the oxygen isotopic records between trees suggests that site-specific and tree-specific conditions dominate this signal. Carbon isotopic records reveal a better correlation between records from multiple trees (r = 0.47, p<0.01) suggesting that a common climate signal is more robustly recorded by the δ13C of these trees. At interannual frequencies (5 year), both carbon and oxygen isotopic records correlate significantly with wet season precipitation (r = -0.50, and -0.55 respectively, p<0.05). Spectral analysis reveals dominant 8-10 year and 3-5 year periodicities in both the carbon and oxygen isotopic records of individual trees. The oldest tree examined reveals a shift from this 8-10 year periodicity during the early part of the record to a lower frequency (20-24 year) variability during the last century. The lower frequency variability identified in the records is associated with both the Pacific Decadal Oscillation and the decadal and multidecadal variability observed in the tropical North Atlantic.

Collectively, these data show that tropical tree ring growth and isotopic composition in the southwestern Amazon basin are precipitation dependent and these measures can be exploited to reconstruct a hydrologic history for this region.

The Amazon-Andes region of tropical South America plays host to the world’s largest river, its highest biodiversity and one of its highest mountain chains, as well as a long and complex history. As such, it represents an ideal location to study the interplay between geology, climate and biodiversity. In this thesis I first present a statistical analysis of the usefulness of paleocurrent data in reconstructing that complex history, using high-resolution topographic data from the Shuttle Radar Topography Mission to develop and test a river network model of the modern Amazon River watershed, followed by new zircon geochronology provenance data from the Alter do Chão Formation in Brazil, with implications for hypothesized Miocene drainage reversal in Amazonia. I then present potentially the first attempt to utilize phylogenetic data to inform our understanding of the uplift history of the Andes, using a simple biogeographic model of vicariant speciation along with phylogenetically-established dates for isolation events to compare varying simulations of Andean uplift over the past 10 million years.

I will conclude (1) that paleocurrent analysis is likely of very limited use in watershed-scale investigations, while highlighting the complexity of river flow patterns within a large basin such as the Amazon, and (2) that the topography found at the modern day Huancabamba Depression likely did not reach its current elevation earlier than ca. 3 million years.

The tropics are a significant source of heat and moisture export, which drive global circulation patterns, thus it is vital to systematically understand the land, ocean and sedimentological interactions within the tropics. The Brazilian continental margin is an ideal region to characterize the tropics due to its unique local oceanography and proximity to the atmospheric engine that is the Amazon Basin. A combination of: 1) terrestrial organics and hydrology; 2) oceanographic temperature, isotopic composition, and salinity and 3) early diagenesis and geochemistry of sedimentary interstitial water and methane hydrate, provide a detailed understanding of the primary constituents that influence the South American tropics.

Sedimentological, organic and paleoceanographic reconstructions of the Amazon Basin, Brazilian Nordeste, and western equatorial Atlantic have been undertaken on two sediment cores located on the Brazilian continental slope representing 30 and 110 ka, respectively. High-resolution XRF analyses of Fe, Ti, K and Ca are used to define the sedimentological history of the Amazon Basin and northern Nordeste. Here we present elemental ratios of Ti/Ca and Fe/K, in addition to magnetic susceptibility, to determine variability in Amazon Basin and Nordeste hydrology. Bulk organic proxies d13C and d15N of sedimentary organic carbon are used to define the organic history of the Amazon Basin. Peaks in Ti/Ca and Fe/K ratios largely correlate in both the Amazon Basin and in the Nordeste Record. These excursions correlate with commonly modeled global slowdown of the Atlantic meridional overturning circulation (AMOC) during increased northern hemisphere glaciation. Differences in the Fe/K and Ti/Ca records suggest periods of increased chemical weathering independent of precipitation driven sediment discharge in the Brazilian Nordeste. Bulk organic geochemistry indicates the vegetative history of the Amazon Basin has been relatively stable during the late Quaternary.

High-­resolution stable oxygen isotopic analysis and Mg/Ca paleothermometry undertaken on the near-­surface-dwelling planktic foraminiferal species Globierinoides ruber provide a picture of paleoceanographic forcings in the western equatorial Atlantic. The Nordeste core exhibits a rapid warming of ~3.5ºC between the last glacial maximum and the early Holocene. Furthermore, in almost all cases during the last glacial stage, there was a 0.5 to 2ºC warming of the western equatorial Atlantic during the periods of high Ti/Ca ratios that correlate with slowdown of AMOC. Thus, as observed in some previous studies, the western equatorial Atlantic was warm and the adjacent southern tropical continent was wet coincident with increased glaciation in the high latitude northern hemisphere.

Interstitial pore waters were analyzed from the Amazon Fan and Brazilian continental slope to determine early diagenesis, methane hydrate potential and its geographic variability. Interstitial waters were measured for total Mg, Ca, SO4, alkalinity, Cl, and d18O, combined with seismic bottom simulating reflectors (BSRs) to investigate early diagenesis. Interpolated maps of sulfate reduction, sedimentation rate and maximum alkalinity were produced to examine geographic variability in early diagenesis and methane hydrates. Inorganic precipitation of calcium and magnesium, likely via dolomite and siderite, correlates with a decrease in alkalinity through inorganic carbonate precipitation and methanogenesis, but alternatively increases through redox pathways, specifically sulfate reduction.

Through multiple lines of evidence it is likely that there is extensive methane hydrate occurrence on the Amazon continental shelf. A combination of: 1) A rapid linear decline in sulfate at an unusually shallow depth; 2) Uncommonly high sedimentation rates and terrestrial organic carbon input; 3) Significant variability in what should be conservative chloride concentrations; 4) Large variability in interstitial oxygen isotopes; 5) Widespread occurrence of BSRs provide substantial support for the presence of methane hydrates. Sulfate reduction rates are lowest along the main Amazon channel, with highest values distal of the main channel. Sedimentation rates are relatively low on the continental shelf and surrounding the main channel and highest toward the distal end of the main channel. Sulfate reduction rates provide key insights to the potential geographic variability of methanogenesis and methane hydrate formation. Given the tremendous influx of sediment from the Amazon River, this region is prone to massive sediment failures, subsequent release of methane hydrates, as well as significant potential for natural gas hydrates.

Results from two geographical study areas are presented in this dissertation, contributing important new information about the Cenozoic geological, environmental and biological regime. At the first locality along the Manu River (11.90°S, 71.34°W) I present a revised age and reinterpret the depositional environment. Age determination consisted of radiometrically dating detrital zircons, pushing the technical boundaries of this technique, paired with radiocarbon analyses of depositional organic material. Constraining the depositional environment relied on geochemical and sedimentological analyses. The revised age, 0.13±0.04 Ma instead of ~9 Ma, and depositional environment, as a fluvial overbank deposit instead of estuarine or marginal-marine deltaic deposits, have major implications for research previously published on the outcrop.

A second section is realized along the Alto Madre de Dios river and is situated in a piggyback basin belonging to the frontmost active deformation zone of the Sub Andean Zone, located approximately 12.8°S and 71.3°W. The section spans from the Paleocene to the Quaternary in time and allows a constraint on the influence of the Andean orogeny on the Madre de Dios foreland basin and the fauna and flora it contained through the Cenozoic. Field work has yielded a series of fossil localities that have been constrained in age through use of U/Pb dating of detrital zircons. Notable fossil finds include Early Miocene marsupials, xenarthrans, rodents, notoungulates, and bats. A new primate recovered from the section is the first record of a primate from the Early Miocene of the Amazon Basin. Reconstruction of the depositional environment for each formation was performed by a combined use of sedimentology, geochemistry, and paleontology. Stable isotopic analyses on depositional organic material indicates continental depositional settings for all formations, whereas stable isotopic analyses on abiogenic carbonates provide constraints on the degree of diagenetic alteration of sediments varying with age and structural setting. Sedimentary facies are consistent with deposits being formed in fluvial and overbank environments, consisting of fine-grained floodplain deposits, point bar deposits, conglomerate channels and fossil-bearing channel lag deposits. Average grain size for the section shows a coarsening trend towards younger deposits, consistent with the approach of, and incorporation into an active orogen, as supported by provenance data from detrital zircon analyses.