Abstract
The Mississippi River Delta (MRD) has undergone tremendous land loss over the past century due to natural and anthropogenic influences, a fate shared by many river deltas globally. A globally unprecedented effort to restore and sustain the remaining subaerial portions of the delta is now underway, an endeavor that is expected to cost $50–100B over the next 50 yr. Success of this effort requires a thorough understanding of natural and anthropogenic controls on sediment supply and delta geomorphology. In the MRD, hurricanes have been paradoxically identified as both substantial agents of widespread land loss, and vertical marsh sediment accretion. We present the first multi-decadal chronostratigraphic assessment of sediment supply for a major coastal basin of the MRD that assesses both fluvial and hurricane-induced contributions to sediment accumulation in deltaic wetlands. Our findings indicate that over multidecadal timescales, hurricane-induced sediment delivery may be an important contributor for deltaic wetland vertical accretion, but the contribution from hurricanes to long-term sediment accumulation is substantially less than sediment delivery supplied by existing and planned river-sediment diversions at present-day river-sediment loads.
Highlights
The Mississippi River Delta (MRD) has undergone tremendous land loss over the past century due to natural and anthropogenic influences[1], a fate shared by many river deltas globally[2]
Mean rates of vertical accretion throughout Breton Sound obtained through analysis of 137Cs in this study (0.77 cm yr−1) are corroborated by previous basinwide studies of vertical accretion in Breton Sound (0.72 cm yr−1)[14] and other regions of the Mississippi River delta plain (0.78 cm yr−1)[16]
Taking the total mineral sediment accumulation in the 518 km[2] study area (31 million metric tons (Mmt)) and dividing it by the average timespan of all sediment cores (64 y) yields an annual regional mineral accumulation rate of 0.48 Mmt y−1 (Table 1). This rate is relatively modest compared to the West Bay diversion, located downstream on the Mississippi River, which receives 6% of total Mississippi flow, and accumulates up to 2 Mmt of sediment during < 1 yr within a 70 km[2] receiving basin[26]
Summary
The Mississippi River Delta (MRD) has undergone tremendous land loss over the past century due to natural and anthropogenic influences[1], a fate shared by many river deltas globally[2]. In the MRD, like many other deltas, sediment supply has been reduced by human activities[1,5], subsidence rates can exceed 25 mm yr−1 11, and sea-level rise is accelerating These combined trends have contributed to the loss of nearly 4000 km[2] of MRD coastal wetlands since 1932, with over 50% of that loss being directly attributed to submergence[12]. Diversions have been implicated in land loss, especially following Hurricane Katrina in 2005, during which over 100 km[2] of wetlands were converted to open water in upper Breton Sound[6], into which the Caernarvon Freshwater Diversion (CFD) has discharged Mississippi River water for over 20 years. This land loss has been linked to diversion inflows of nutrient-rich river water, associated with weakened soils and plant growth[17,18,19]
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