Juan L. Gonzalez

Deciphering Holocene sea-level history on the U.S. Gulf Coast: A high-resolution record from the Mississippi Delta

Published Holocene relative sea-level (RSL) curves for the U.S. Gulf Coast are in mutual conflict, with some characterized by a smooth RSL rise akin to widely accepted eustatic sea-level curves versus others, including several recent ones, that are characterized by a conspicuous “stair-step” pattern with prolonged (millennium-scale) RSL stillstands alternating with rapid (meter-scale) rises. In addition, recent work in Texas and Alabama has revitalized the notion of a middle Holocene RSL highstand, estimated at 2 m above present mean sea level. An extensive sampling program in the Mississippi Delta (Louisiana) focused on the collection of basal peats that accumulated during the initial transgression of the pre-existing, consolidated Pleistocene basement. We used stable carbon isotope ratios to demonstrate that many of these samples accumulated in environments affected by frequent saltwater intrusion in the <30 cm zone between mean spring high water and mean sea level, and we selected plant macrofossils that were subjected to AMS 14C dating. Nearly 30 sea-level index points from a ∼20 km2 study area on the eastern margin of the delta suggest that RSL rise followed a relatively smooth trend for the time interval 8000–3000 cal yr B.P., thus questioning the occurrence of major RSL stillstands alternating with abrupt rises. Given the narrow error envelope defined by our data set, any sea-level fluctuations, if present, would have amplitudes of <1 m. Although a true middle Holocene highstand never occurred in the Mississippi Delta, the high level of detail of our time series enables a rigorous test of this hypothesis. Correction of our data set for a hypothetical tectonic subsidence rate of 1.1 mm yr−1 (assuming a constant subsidence rate compared to the tectonically relatively stable adjacent coast of Texas) leads to sea levels near 2 m above present during the time interval 6000–4000 cal yr B.P. However, this model also implies a RSL position near −2 m around 8000 cal yr B.P., which is inconsistent both with data of this age from Texas, as well as with widely accepted sea-level data from elsewhere. We therefore conclude that a middle Holocene highstand for the U.S. Gulf Coast is highly unlikely, and that the entire area is still responding glacio-isostatically, by means of forebulge collapse, to the melting of the Laurentide Ice Sheet.

Coastal Louisiana in crisis: Subsidence or sea level rise?

The drowning of wetlands and barrier islands in coastal Louisiana has become a widely publicized environmental catastrophe in the wake of hurricanes Katrina and Rita in 2005. The devastation caused by these storms has reenergized the debate about restoring the natural coastal-defense system and building higher and sturdier levees, in anticipation of future storms. Understanding the contributions of land subsidence and eustatic (global) sea level rise to Louisianas wetland loss is crucial to the success of any plan designed to protect coastal communities. It is argued here that accelerated sea level rise in the future may pose a larger threat than subsidence for considerable portions of coastal Louisiana.

Morphodynamics of a high discharge tropical delta, San Juan River, Pacific coast of Colombia

Abstract The San Juan River has one of the most extensive and best developed deltas on the Pacific coast of South America, measuring 800 km 2 . The river drainage basin measures 16 465 km 2 and is located in one of the areas with the highest precipitation in the western hemisphere. The annual rainfall varies from 7000 to 11 000 mm, and as a result the San Juan River has the highest water discharge (2550 m 3 s −1 ), sediment load (16×10 6 t yr −1 ), and basin-wide sediment yield (1150 t km −2 yr −1 ) on the west coast of South America. The San Juan delta growth began approximately 5000 years BP. The structure of the delta is determined by the interactions between fluvial deposition and the effect of 1.7-m significant swells, mostly from the SW, and strong tidal currents. Analysis of delta progradation indicates that during 1848–1992 the morphology of the delta was characterized by beach ridge accretion, spit growth, narrowing of inlets, and a general advance of the delta shoreline. During the past decade processes such as rapid erosion of the delta shore, narrowing of barrier islands, and breaching of a new inlet, are the result of a long-term relative sea-level rise of 2.6 mm yr −1 due to tectonically induced subsidence coupled with a eustatic rise of sea-level. The delta also experiences strong oceanographic manifestations associated with the El Nino–La Nina cycle, causing regional sea-level elevation of 20–30 cm during El Nino years. Recent coastal subsidence in the delta is evidenced by: (1) increased occurrence of non-storm washover events; (2) increased erosion of barrier islands with average loss of 11 m yr −1 during 1993–1997; and (3) a relative sea-level rise of 3.4 mm yr −1 during 1991–1999. The morphology and recent evolution of the San Juan delta are unique when compared to other deltas of South America because of the singular combination of extreme climatic, geologic, and oceanographic conditions under which the delta has formed and the absence of human-induced impact in the drainage basin.

Coastal erosion and village relocation: a Colombian case study

Because of its tectonic setting, the Pacific coast of Colombia is subject to a variety of geological hazards, including earthquakes, tsunamis and associated phenomena such as regional and local coastal subsidence, flooding and soil liquefaction. Erosional trends are prevalent along much of the 700 km long, low barrier islands shorelines of the Pacific littoral and land losses are enhanced by factors such as 30 cm regional sea level rises associated to the occurrence of El Nino. Marine erosion is threatening more seashore littoral villages and worsening the already difficult socioeconomic conditions of most part of the inhabitants. Because of diverse and strong motivations to stay near the sea, the responses of barriers islands inhabitants to marine erosion has consisted in most cases of repetitive in-shore and along-shore directed relocations of villages, rather than definitive abandonment of the islands. In the long run, this procedure only has postponed the problem and led to repetitive relocations and economical losses. The recent inland relocation of El Choncho village, on the San Juan River delta, illustrates a different response to marine erosion. Although a new along-shore relocation was physically possible, inhabitants decided to abandon the barrier island and migrate to an interior, ancient beach ridge complex, applying a prudent solution which will be the most appropriate for other threatened villages of the Pacific littoral. A detailed geomorphologic mapping program must be conducted in order to identify appropriate sites for inland relocation of existing villages on the barriers islands of the Colombian Pacific coast.

The 1 April 2014 Pisagua tsunami: Observations and modeling

On 1 April 2014, an earthquake with moment magnitude Mw 8.2 occurred off the coast of northern Chile, generating a tsunami that prompted evacuation along the Chilean coast. Here tsunami characteristics are analyzed through a combination of field data and numerical modeling. Despite the earthquake magnitude, the tsunami was moderate, with a relatively uniform distribution of runup, which peaked at 4.6 m. This is explained by a concentrated maximal slip at intermediate depth on the megathrust, resulting in a rapid decay of tsunami energy. The tsunami temporal evolution varied, with locations showing sustained tsunami energy, while others showed increased tsunami energy at different times after the earthquake. These are the result of the interaction of long period standing oscillations and trapped edge wave activity controlled by inner shelf slopes. Understanding these processes is relevant for the region, which still posses a significant tsunamigenic potential.

Development of an Operational Nearshore Wave Forecast System for Puerto Rico and the U.S. Virgin Islands

Abstract Anselmi-Molina, C.M.; Canals, M.; Morell, J.; Gonzalez, J.; Capella, J., and Mercado, A, 2012. Development of an operational nearshore wave forecast system for Puerto Rico and the U.S. Virgin Islands. An assessment of coastal information needs in Puerto Rico and the U.S. Virgin Islands identified real-time wave data and improved wave forecasts as essential for supporting nearshore operations in the region. After deployment of operational wave-measuring buoys off the north, south, east, and west coasts of the Puerto Rico–Virgin Islands archipelago, the Simulating WAves Nearshore (SWAN) model was implemented for the region. The main purpose of its implementation is to provide for data needs in areas not well represented by the buoys as well as for filling the nearshore gap not addressed by the operational National Oceanic and Atmospheric Administration (NOAA) Multigrid WAVEWATCH III (NMWWIII) model forecast. The model consists of a nesting between WWIII and SWAN where oceanic boundary conditions in the form of two-dimensional wave spectra are supplied by the NMWWIII model. In this study we present results from two model-validation experiments under varying physical-forcing scenarios, one in which swell waves dominate (Rincón) and one in which wind swell dominates (Ponce). The results show good agreement between the observed and the modeled wave field for the Rincón domain, on Puerto Ricos northwest coast and facing the Atlantic Ocean, which indicates that appropriate boundary conditions are being obtained from NMWWIII and propagated correctly by SWAN into nearshore areas. Results for the Ponce domain, in Puerto Ricos southern coast and facing the Caribbean Sea, highlight the need for more accurate high-resolution wind products to properly resolve the wind-driven wave field dominating this area.

Reconstructing “background” Rates of sea‐level rise as a tool for forecasting coastal wetland loss, Mississippi Delta

The Mississippi Delta is one of the most vulnerable coastal regions in the world, with rapidly deteriorating wetlands and an increasing threat for the city of New Orleans due to accelerated relative sea-level (RSL) rise. Rational coastal forecasting and policy-making for this area requires a detailed understanding of the temporal and spatial dimensions of RSL change. However, considerable controversy currently exists about the nature of Holocene RSL rise along the Gulf Coast. We have collected new, high-resolution RSL data from the eastern part of the Mississippi Delta that show that this area experienced smooth and continuous RSL rise that gradually decreased during the Holocene, consistent with numerous observations worldwide. Here, we demonstrate the potential of such data for quantifying natural “background” rates of RSL rise that should be taken into account in coastal forecasting of such threatened environments.

Coseismic slip and afterslip of the 2015 Mw 8.3 Illapel (Chile) earthquake determined from continuous GPS data

We analyzed the coseismic and early postseismic deformation of the 2015, Mw 8.3 Illapel earthquake by inverting 13 continuous GPS time series. The seismic rupture concentrated in a shallow (<20 km depth) and 100 km long asperity, which slipped up to 8 m, releasing a seismic moment of 3.6 × 1021 Nm (Mw = 8.3). After 43 days, postseismic afterslip encompassed the coseismic rupture. Afterslip concentrated in two main patches of 0.50 m between 20 and 40 km depth along the northern and southern ends of the rupture, partially overlapping the coseismic slip. Afterslip and aftershocks confined to region of positive Coulomb stress change, promoted by the coseismic slip. The early postseismic afterslip was accommodated ~53% aseismically and ~47% seismically by aftershocks. The Illapel earthquake rupture is confined by two low interseismic coupling zones, which coincide with two major features of the subducting Nazca Plate, the Challenger Fault Zone and Juan Fernandez Ridge.

Earthquake damage assessment for deterministic scenarios in Iquique, Chile

Risk evaluation and loss analysis is key in foreseeing the impact of disasters caused by natural hazards and may contribute effectively in improving resilience in a community through the pre-evaluation of preparedness and mitigation actions. The pilot study presented herein is for the Chilean city of Iquique, which is located at the core of a seismic gap that extends from south Perú to north Chile, and has strategic geopolitical and economic importance for the country. The region was hit April 1, 2014, by an