Ian Bailey

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

The Breakdown of Static and Evolutionary Allometries during Climatic Upheaval

The influence of within-species variation and covariation on evolutionary patterns is well established for generational and macroevolutionary processes, most prominently through genetic lines of least resistance. However, it is not known whether intraspecific phenotypic variation also directs microevolutionary trajectories into the long term when a species is subject to varying environmental conditions. Here we present a continuous, high-resolution bivariate record of size and shape changes among 12,633 individual planktonic foraminifera of a surviving and an extinct-going species over 500,000 years. Our study interval spans the late Pliocene to earliest Pleistocene intensification of northern hemisphere glaciation, an interval of profound climate upheaval that can be divided into three phases of increasing glacial intensity. Within each of these three Plio-Pleistocene climate phases, the within-population allometries predict evolutionary change from one time step to the next and that the within-phase among-population (i.e., evolutionary) allometries match their corresponding static (within-population) allometries. However, the evolutionary allometry across the three climate phases deviates significantly from the static and phase-specific evolutionary allometries in the extinct-going species. Although intraspecific variation leaves a clear signature on mean evolutionary change from one time step to the next, our study suggests that the link between intraspecific variation and longer-term micro- and macroevolutionary phenomena is prone to environmental perturbation that can overcome constraints induced by within-species trait covariation.

Temperature is a poor proxy for synergistic climate forcing of plankton evolution

Changes in biodiversity at all levels from molecules to ecosystems are often linked to climate change, which is widely represented univariately by temperature. A global environmental driving mechanism of biodiversity dynamics is thus implied by the strong correlation between temperature proxies and diversity patterns in a wide variety of fauna and flora. Yet climate consists of many interacting variables. Species probably respond to the entire climate system as opposed to its individual facets. Here, we examine ecological and morphological traits of 12 633 individuals of two species of planktonic foraminifera with similar ecologies but contrasting evolutionary outcomes. Our results show that morphological and ecological changes are correlated to the interactions between multiple environmental factors. Models including interactions between climate variables explain at least twice as much variation in size, shape and abundance changes as models assuming that climate parameters operate independently. No dominant climatic driver can be identified: temperature alone explains remarkably little variation through our highly resolved temporal sequences, implying that a multivariate approach is required to understand evolutionary response to abiotic forcing. Our results caution against the use of a ‘silver bullet’ environmental parameter to represent global climate while studying evolutionary responses to abiotic change, and show that more comprehensive reconstruction of palaeobiological dynamics requires multiple biotic and abiotic dimensions.