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. O rogenesis reflects the balance of crustal material entering a mountain belt to undergo shortening and uplift versus material leaving the orogen through exhumation, erosion, and sediment transport (1-5). Perturbations in the influx/efflux from the orogen are expected to result in predictable changes in deformation within the orogen as it attempts to reestablish equilibrium (3). The long-term sink for sediment transported out of mountain belts is often in the deep sea, particularly in large submarine fans where sediments accumulate at anomalously high rates (>10 cm/ky) compared with deep-sea pelagic sedimentation (6-8). Even higher sedimentation rates (>100 cm/ky) proximal to glacially eroded regions (9-14) imply that wet-based glaciers are extremely efficient agents of erosion. Observations and modeling have argued that erosion rates can influence tectonic processes (15)(16)(17)(18)(19), but the timescales of adjustment, and the role of landscape disequilibrium, remain unclear. For example, exceptionally high local sedimentation rates (100-1000 cm/ky) recorded on the century timescale (13) SignificanceIn 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...

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Magnetotactic bacteria (MTB) synthesize magnetite and greigite crystals under low oxygen conditions in the water column or uppermost sediment (greigite‐producing bacteria are found below the oxic‐anoxic transition). Dissolved iron and oxygen contents in local environments are known to be limiting factors for the production and preservation of biogenic magnetite. Understanding the processes that link MTB to their living environments is fundamental to reconstructing past chemical variations in the water column and sediment, and for using the magnetic properties of biogenic magnetite as environmental proxy indicators. Previous studies have suggested that the frequently identified biogenic soft (BS) and biogenic hard (BH) magnetite types are associated with equant and more elongated morphologies, respectively, and that their abundance varies in accordance with sedimentary oxygen content, where MTB that produce the BH component live in less oxygenated environments. We test this hypothesis in a high‐resolution integrated environmental magnetic and geochemical study of surface sediments from Mamanguá Ría, SE Brazil. Based on magnetic and pore water profiles, we demonstrate that both the BS and BH components occur within microaerobic environments and that as sediment oxygen content decreases with depth, the BS component disappears before the BH component. With continued burial into the sulfidic diagenetic zone, both components undergo progressive dissolution, but the BH component is more resistant to dissolution than the BS component. Our observations confirm previous inferences about the relative stability of these phases and provide a firmer basis for use of these two types of biogenic magnetite as paleoenvironmental proxies.

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