Geology | 2019
Ice cover as a control on the morphodynamics and stratigraphy of Arctic deltas
Abstract
Deltas are dynamic systems that can provide important information on past climate conditions. Arctic deltas have the potential to preserve information about climate in one of the most temperature-sensitive regions of the Earth. We present experimental results assessing the effects of ice cover on delta morphodynamics to identify signatures of ice-cover presence during deposition. Ice cover drives spatial variation in sediment transport on the subaqueous delta clinoform through sub-ice channels, which leads to the development of (1) extended delta lobes built by elongated, subaqueous sediment wedges and (2) bathymetry with increasing topographic roughness from the shoreline to a depth ≈ bottom-fast ice thickness. These unique seascape and stratigraphic features record past climate conditions, and can serve as indicators of climate change on vulnerable Arctic coasts. INTRODUCTION Deltas respond to coastal processes, river dynamics, and atmospheric conditions, and can record environmental conditions, such as relative sea level, sediment supply, and climate (HellandHansen and Martinsen, 1996; Walker, 1998; Hill et al., 2001; Kim et al., 2006; Bianchi and Allison, 2009). Arctic deltas may preserve information about climate and climate change because the Arctic is sensitive to temperature changes due to the presence of the cryosphere (Walker, 1998; Walker and Hudson, 2003; Rowland et al., 2010). Arctic deltas that interact with shore-fast ice are thought to share a distinctive morphology characterized by an extended, low-slope subaqueous topset and an abrupt slope break a few meters below sea level where ice becomes ungrounded (Reimnitz and Bruder, 1972; Dupré, 1980; Are and Reimnitz, 2000; Hill et al., 2001; Reimnitz, 2002). However, Arctic deltas have largely been neglected as records of climate (Bianchi and Allison, 2009), and it is unclear whether the suggested subaqueous morphology is a unique characteristic of Arctic deltas and a result of sediment emplacement beneath shorefast ice (Are and Reimnitz, 2000). Given the importance of Arctic deltas as landscape-scale climate records and the lack of understanding of how ice cover controls their morphology, we performed laboratory experiments to evaluate the controls of ice cover on delta morphodynamics and associated depositional processes. Our work is aimed at testing the following hypotheses: (1) ice cover extends subaqueous topset deposits through sub-ice channels, (2) interaction between ice cover and sub-ice channels results in distinct subaqueous delta morphology and stratigraphy, and (3) sub-ice channels transport coarse sediments farther basinward. EXPERIMENTAL DESIGN We conducted two pairs of experiments in which deltas were constructed in ice-covered and ice-free basins under different sediment discharges (see the Data Repository1 and Figure DR1 and Table DR1 therein, for the experimental setup). Sediment and water were fed at a constant ratio into the basin with a constant water level. We used a 2:1 mixture (by volume) of brown crushed walnut shells (diameter D = 74–250 μm, density ρ = 1.3 g/cm3) and white quartz sand (D = 170– 200 μm, ρ = 2.65 g/cm3), which serve as proxies for fineand coarse-grained sediments, respectively (Kim et al., 2006; Martin et al., 2009). For experiments under ice-free conditions, we used water at ~19.5 °C, and for experiments with ice cover, the water temperature was ~3 °C (see the Data Repository). Because the geomorphic work on ice-covered deltas is associated with peak annual discharge, sediment supply, and flooding that occur around the spring breakup of Arctic rivers (Reimnitz and Bruder, 1972; Walker, 1998; Hill et al., 2001; Reimnitz, 2002; Walker and Hudson, 2003), our experiments simulate this ice-covered period of activity. We do not test seasonality (i.e., seasonal changes in ice cover) nor ice-breakup discharge, but focus on the morphodynamic effects of ice-cover presence or absence. To mimic shore-fast ice typical in the Arctic (Are and Reimnitz, 2000), a floating cover of pressed crushed ice with a uniform thickness of ~7 cm was placed at the upstream end of the experiment tank, which became frictionally coupled to the growing delta surface during deposition (Fig. DR1). Overhead time-lapse photography captured delta morphology, and a second camera beneath the tank’s glass bottom captured delta toe progradation. At the end of each experiment, we drained the basin to examine the resultant delta morphology (Figs. 1A and 1B). To study the deposit stratigraphy, half of each deposit was vertically sectioned in the radial direction (Figs. 2A–2C) and the other half was horizontally sectioned in 1 cm increments down from the initial water level. All experiment images were orthorectified and analyzed using Adobe Photoshop® software (Figs. DR2–DR6). EXPERIMENTAL RESULTS AND INTERPRETATION Planform Patterns Experimental ice-covered deltas had larger subaqueous extents than ice-free deltas (Fig. 1) as hypothesized in, e.g., Reimnitz and Bruder 1GSA Data Repository item 2019134, physical experimental data and natural delta bathymetric data, is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Lim, Y., et al., 2019, Ice cover as a control on the morphodynamics and stratigraphy of Arctic deltas: Geology, v. 47, p. 399–402, https:// doi .org /10 .1130 /G45146.1 *E-mail: yjlim322@ utexas .edu Manuscript received 11 May 2018 Revised manuscript received 7 February 2019 Manuscript accepted 8 February 2019 https://doi.org/10.1130/G45146.1 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 6 March 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/5/399/4680693/399.pdf by Univ of Texas-Austin user on 22 October 2019 400 www.gsapubs.org | Volume 47 | Number 5 | GEOLOGY | Geological Society of America (1972). Radial distances from the shoreline to delta toe were ~1.5×–3× larger for deltas formed beneath ice cover than for ice-free deltas (Figs. 1C and 1D). Because the proximal subaqueous cross-sectional area was limited for aggradation due to the ice cover, sediment advanced below the ice and farther basinward, promoting subaqueous topset extension through delta toe progradation. Similarly, ice cover restricted shoreline progradation and promoted subaerial aggradation, producing a smaller subaerial topset area in plan view. The planform patterns of the shoreline and delta toe were also rougher for ice-covered deltas than for ice-free deltas (Fig. 1). Ice cover induced spatially varying subaqueous, underice sediment transport and created (1) localized elongated lobate features (hereafter “lobes”) on the foreset, and (2) areas between the lobes (hereafter “inter-lobes”) that had significantly less basinward growth. This resulted in irregular toe lines with high rugosity (Figs. 1C and 1D). In contrast, ice-free deltas showed smooth and radially symmetric patterns as a result of a uniform shoreline progradation due to freely migrating channels. The ratio of ice-covered to ice-free subaqueous topset lengths (Figs. 1C and 1D) is therefore a geomorphic marker for icedelta interaction (Figs. DR7 and DR8). Subsurface Architecture Subsurface architecture was also affected by spatially varying sediment transport and deposition beneath the ice cover (Fig. 2). Sections through ice-covered deltas showed discrete, low-angle sand (coarse-sediment proxy) deposits farther basinward in the lobes (Figs. 2A and 2D), which were much less apparent in the inter-lobes (Figs. 2B and 2D). Measurements of subaqueous sand thickness normalized by the total subaqueous deposit thickness (Fig. 2E) showed sand deposits in a lobe extending ~50 mm farther basinward than in an inter-lobe. Sections of ice-free deltas, however, were essentially uniform in all radial directions and exhibited no such sand deposits (Fig. 2C) as indicated by proportionally lower subaqueous sand thickness compared to ice-covered deltas (Fig. 2E). In addition, subaerial deposit and subaerial sand thicknesses of ice-covered deltas were higher than in the ice-free deltas, and were slightly lower at the lobe than at the inter-lobe (Fig. DR9). Dynamics and Time Scale of Sub-Ice Channelization Process The subaqueous, near-horizontal sand layers in ice-covered deltas prominently identified at the lobes (Fig. 2A) provide evidence of experimental sub-ice channel formation. Without local, channelized flows under the ice cover, experimental subaqueous sand transport should be dominated by grain avalanche and result in Radial direction [degrees] S ho re lin e to to e di st an ce [m m ]