Daniel P. Maxbauer

MAX UnMix

It is common in the fields of rock and environmental magnetism to unmix magnetic mineral components using statistical methods that decompose various types of magnetization curves (e.g., acquisition, demagnetization, or backfield). A number of programs have been developed over the past decade that are frequently used by the rock magnetic community, however many of these programs are either outdated or have obstacles inhibiting their usability. MAX UnMix is a web application (available online at http://www.irm.umn.edu/maxunmix), built using the shiny package for R studio, that can be used for unmixing coercivity distributions derived from magnetization curves. Here, we describe in detail the statistical model underpinning the MAX UnMix web application and discuss the programs functionality. MAX UnMix is an improvement over previous unmixing programs in that it is designed to be user friendly, runs as an independent website, and is platform independent. HighlightsOnline application for unmixing magnetic coercivity distributions.User-friendly interface increases accessibility.Results from MAX UnMix are comparable with existing methods.

High Arctic forests during the middle Eocene supported by moderate levels of atmospheric CO2

Fossils from Paleogene High Arctic deposits provide some of the clearest evidence for greenhouse climates in the past and offer the potential to improve our understanding of Earth system dynamics in a largely ice-free world. One of the most well-known and stunningly preserved polar forest sites, Napartulik, crops out of middle Eocene (47.9–37.8 Ma) sediments on eastern Axel Heiberg Island, Nunavut, Canada (∼78°N paleolatitude). An abundance of data from Napartulik suggests mean annual temperatures at least 30 °C warmer than today and atmospheric water loads 2× current levels. Despite this wealth of paleontological and paleoclimatological data, there are currently no direct constraints on atmospheric CO 2 levels for Napartulik or any other polar forest site. Here we apply a new plant gas-exchange model to Metasequoia (dawn redwood) leaves to reconstruct atmospheric CO 2 from six fossil forest horizons at Napartulik. Individual reconstructions vary between 392 ppm and 474 ppm, with a site median of 424 ppm (351–523 ppm at 95% confidence). These estimates represent the first direct constraints on CO 2 for a High Arctic forest and suggest that the temperate conditions present at Napartulik during the middle Eocene were maintained under CO 2 concentrations ∼1.5× pre-industrial levels. Our results support the case that long-term climate sensitivity to CO 2 in the past was sometimes high, even during largely ice-free periods, highlighting the need to better understand the climate forcings and feedbacks responsible for this amplification.

Response of pedogenic magnetite to changing vegetation in soils developed under uniform climate, topography, and parent material

Pedogenesis produces fine-grained magnetic minerals that record important information about the ambient climatic conditions present during soil formation. Yet, differentiating the compounding effects of non-climate soil forming factors is a nontrivial challenge that must be overcome to establish soil magnetism as a trusted paleoenvironmental tool. Here, we isolate the influence of vegetation by investigating magnetic properties of soils developing under uniform climate, topography, and parent material but changing vegetation along the forest-prairie ecotone in NW Minnesota. Greater absolute magnetic enhancement in prairie soils is related to some combination of increased production of pedogenic magnetite in prairie soils, increased deposition of detrital magnetite in prairies from eolian processes, or increased dissolution of fine-grained magnetite in forest soils due to increased soil moisture and lower pH. Yet, grain-size specific magnetic properties associated with pedogenesis, for example relative frequency dependence of susceptibility and the ratio of anhysteretic to isothermal remanent magnetization, are insensitive to changing vegetation. Further, quantitative unmixing methods support a fraction of fine-grained pedogenic magnetite that is highly consistent. Together, our findings support climate as a primary control on magnetite production in soils, while demonstrating how careful decomposition of bulk magnetic properties is necessary for proper interpretation of environmental magnetic data.

Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal-bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)

Sediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a forearc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3 – 7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coal-bearing unit (∼2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very low-coercivity minerals with unblocking temperatures between 350-580°C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300-400°C), higher ARM, higher frequency dependence, and isotopically heavy δ56Fe across a range of lithologies in the coal-bearing unit (between 1925-1995 mbsf), indicate the presence of fine-grained authigenic magnetite. We suggest that iron-reducing bacteria facilitated the production of fine-grained magnetite within the coal-bearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply iron reducing bacteria in the surrounding siliciclastic sediments. These results indicate that coal-bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere.