1. Introduction
Between the months of May and November, from 25 to 30 tropical depressions may originate each year offshore western Mexico, near Acapulco and between 15 and 20 degrees north of the equator. Depending on sea surface temperatures (SSTs), the strength of such disturbances often reaches hurricane intensity. Most such storms expire harmlessly in the open Pacific Ocean, but some migrate directly northward to enter the Gulf of California, especially during the El Niño intervals that occur every 6 or 8 years; these intervals cause the ocean water off western Mexico to become unusually warm. Even fewer of the storms make a sharp turn to the east to make landfall on the coast of mainland Mexico. The 2015 hurricane season was unusually active [
1] and Hurricane Patricia rapidly intensified to a Category 5 event with wind speeds reaching 346 km/h accompanied by a minimum atmospheric pressure of 872 mbar. It remains the most powerful storm yet recorded in the Western Hemisphere and the strongest to impact western Mexico. Having a maximum diameter of 2400 km, the storm’s outer wind bands swept across the tip of the Baja California peninsula before the eye of the storm turned unexpectedly to the east to strike the village of Cuixmala, which is well below the opening to the Gulf of California. More extensive damage would certainly have resulted had Hurricane Patricia followed a more northernly direction over the warm waters in the Gulf of California. A year earlier, in September 2014, Hurricane Odile became one of the most destructive storms to damage infrastructure on the peninsula [
2].
At the center of the Category 4 Hurricane Odile, the eyewall struck Cabo San Lucas with a sustained wind speed of 144 km/h.Tracking farther into the Gulf of California, when the hurricane reached the town of Loreto (located 375 km farther north) later that day, the maximum wind speed fell to 113 km/h. Under the influence of global warming, it is a question of not if, but when, a hurricane with strength equal to or greater than Odile will next impact shores around the Gulf of California.
A solid geologic record in Mexico’s Baja California area shows the passage of major storms over the past 10,000 of the Holocene Era, and even as far back as 125,000 years ago, during the last interglacial interval of the Pleistocene Epoch. Late Pleistocene coral reefs on Isla Cerralvo, east of La Paz, exhibit repeated burial events by cobbles attributed to storms, followed by recolonization [
3]. Farther north, within the Gulf of California, the smaller Isla San Diego exhibits a thick accumulation of eroded granodiorite boulders that include Pleistocene marine fossils, but no intact reef structure [
4]. Coastal boulder beds related to Holocene storm events are recognized even farther north, in the Loreto area at Puerto Escondido [
5], Isla del Carmen [
6], and at Ensenada Almeja [
7]. Farther within the upper gulf region on Isla San Luis Gonzaga, a complex set of multiphase boulder deposits are also attributed to Holocene storm events [
8].
Among these and similar examples within the Gulf of California, evidence for the greatest disturbance of a rocky shoreline is found at Arroyo Blanco, on the eastern coast of Isla del Carmen, where huge slabs of limestone were lifted from bedded layers at the seaward edge of a Pleistocene marine terrace and turned up onto their edges. The largest such block has crude dimensions of 5.4 m by 4 m and a bedding thickness of 1 m amounting to a volume of 15 cubic meters and an estimated weight of 28 metric tons based on the density of limestone [
6].
Given that future storms will certainly impact the coastline at these same locations, where coastal boulder deposits are already known to exist [
3,
4,
5,
6,
7,
8], the challenge is to revisit a location soon after the passage of a major storm in order to document the extent to which additional erosion has occurred and determine whether boulders of any previously registered size have shifted in position. Following documentation of investigations in 2018 on Isla del Carmen [
6], the earliest opportunity to survey the physical aftermath of a major storm in that area was Hurricane Kay. Striking the outer shores of the peninsula on 8 September 2022, as a Category 2 event, Hurricane Kay attained wind speeds of 165 km/h and brought heavy rain that broke a long drought on the peninsula before eventually reaching California on the US side of the international border [
9]. Although the eye of the storm never crossed into the Gulf of California, the counter-clockwise rotation of the storm reached across the peninsula to generate wind and strong surf against the gulf coast. The purpose of this report is to seek evidence for any disturbances from Hurricane Kay on Isla del Carmen, where several boulders in the coastal deposit at Arroyo Blanco have profiles that make them readily identifiable. In addition, the survey looked for other kinds of physical alterations to the terrace as well as any biological impacts.
Marginal seas elsewhere in the Pacific Ocean basin along the coast of the Philippines and mainland China suffer typhoons that impact shorelines and do significant damage to infrastructure [
10,
11]. Islands in Mexico’s Gulf of California have little or no infrastructure, but are protected as biosphere reserves. Long-term monitoring of physical erosion around these islands is relevant to the study of coastal geomorphology. The same care should be taken to monitor changes in the natural coastlines throughout Asia that are vulnerable to large storms.
2. Geographical and Geological Setting
Located between the Mexican mainland and the Baja California peninsula, the Gulf of California is a marginal sea with a distinctly elongated shape that extends along a NW–SE axis for 1100 km with a semi-enclosed area of 210,000 km (
Figure 1a). The north end is closed by the delta of the Colorado River, but there is access from the Pacific Ocean at the delta’s south end, where an opening of 180 km (the gulf’s maximum width) admits normal sea water. A range of oceanographic phenomena occurs through this opening, including patterns of seasonal upwelling that enhances biological productivity and supercharges species diversity [
12]. On a seasonal basis, from late November to April, winter winds originate in the north and flow southward to generate large-scale wave trains capable of traversing the entire length of the gulf. Lighter winds during the spring and summer season change direction to blow fromthe south in accordance with a semi-monsoonal pattern of regional atmospheric circulation that brings modest rainfall [
13]. The annual rains have a greater impact in the south than in the north, where they are insufficient to moderate the extreme desert aridity. Based on a long-term archaeological survey on Isla Ángel de la Guarda, Bowen [
14] reported that the large northern island has no permanent source of water and that bedrock pools are replenished mainly by episodic hurricanes bringing a deluge of rainfall. Several years may pass between the arrival of successive storm events, meaning that the island is left largely without fresh water over long intervals of time.
Geologically, the coastline stretching along the western margin of the gulf on the Baja California peninsula is dominated by rocky shores [
15], including those of islands such as Isla del Carmen (
Figure 1b). Among the 40 named islands in the Gulf of California that are large enough to support local fauna and flora, Isla del Carmen is the fourth largest in size, with an area of 143 km
2 [
16]. Overall, the island’s coastal geology is dominated by andesite, a volcanic rock that forms from surface flows. Pliocene and Pleistocene limestone deposits created between 4.5 and 1.8 million years ago occur in scattered localities around the periphery of the island, most notably over a lateral distance of 1.5 km represented by a succession of four marine terraces on the east shore at Arroyo Blanco. The lowest terrace closest to the present shore is the 12 m terrace that sat at sea level approximately 125,000 years ago [
17]. Conditions under which flat-lying limestone layers were pried loose from the outer edge of the uplifted terrace and transported landward by as much as 25 m are attributed to a succession of Holocene hurricanes that occurred over the last 10,000 years (6). Several of the largest limestone slabs that sit loose on the terrace are distinctive in profile. An important question is to what extent could contemporary storms further shift such blocks and otherwise contribute to ongoing coastal erosion.
3. Materials and Method
The coastal boulder deposit (CBD) on the elevated 12 m terrace at Arroyo Blanco on Isla del Carmen was first identified during a visit in March 2016. This site was revisited in February 2018, during which data were collected in preparation for a published report [
6]. No substantial changes in the nature of the CBD were found to have occurred during the intervening two years. Travel restrictions related to the COVID-19 pandemic caused a delay in returning to the site after the passage of Category 2 Hurricane Olaf in September 2021. With the passage of Category 2 Hurricane Kay on 8 September 2022, an opportunity was taken to revisit the site at Arroyo Blanco in order to check for possible changes to the structure of the 12 m terrace, as well as any collateral damage of a biological nature. The tracks of the two storms with landfalls on the outer Pacific shores of the Baja California peninsula were very similar. A reconnaissance trip to Arroyo Blanco took place on 30 September 2022, two weeks after the storm, and follow-up visits to conduct a more detailed survey occurred on 12 and 13 October 2022.
3.1. Onshore Data Collection
Extensive field photos were taken to record aspects of physical erosion as well as biological issues related to the desiccation of land plants that were growing on the limestone platform and the spread of marine organisms ripped from the seabed below and spread as detritus across the same area. Dead bushes were easy to recognize because of their brown color in the absence of any chlorophyl. The main plant affected in this way was the Gulf Star Violet (Stenotis mucronate). The most apparent phenomenon relevant to disturbance of marine life was in connection to the brightly colored, red-orange sea fan belonging to the species Pacifigorgia adamsi. Using a Nikon CoolPix W300TM (Nikon-products, Japan), photos were made showing a large number of desiccated plants and 44 sea fans across a surface area of roughly 6000 m2. Among these, the location on the platform of a half dozen representative land plants and a dozen of the largest sea fans were established by latitude and longitude and mapped. Two kinds of measurements were recorded at 18 spots on the terrace platform. The distance of a given biological feature to the seaward lip of the platform was measured using a 9 m rope that could be extended in marked stages to the closest edge of the platform. The map location and elevation above mean sea level of a given biological feature were recorded using a Global Positioning System Gamin Etrex 22X TM unit (USA production headquarters in Olathe, Kansas). In addition, standing pools of seawater were photographed, some of which had begun to precipitate salt crystals after more than a month of exposure under the sun. Finally, the survey concluded with notes and photographic evidence of physical erosion.
3.2. Subtidal Observations
Because sea fans were detached from their anchorage on the rocky seabed adjacent to the 12 m limestone terrace, it was necessary to investigate the shallowest water depth at which the same gorgonians could be found living in place. This was accomplished by diving to the seabed and photographing what could be found there. The same kind of camera that had been used ashore was employed for this purpose.
5. Discussion
Hurricanes, as they are called in the Atlantic and eastern Pacific basins, are atmospheric disturbances of extraordinary violence fueled by elevated sea-surface temperatures (SSTs) that surpass 26.6 °C in subtropical regions to spawn cyclonic depressions on a seasonal basis [
25]. The Saffir–Simpson Hurricane Wind Scale is a 1 to 5 rating based on a storm’s sustained wind speed [
26]. Winds generated by a rotating system in a Category 2 storm such as Hurricane Kay achieve wind speeds between 154–177 km/h and are considered extremely dangerous, but a Category 3 storm surpassing 178 km/h in wind speed is expected to cause devastating damage. Category 4 and 5 storms with wind speeds that exceed 209 km/h and 252 km/h, respectively, can cause catastrophic damage.
The present study follows in the tradition of investigations pioneered by Ball et al. from 1967 [
27], in which parts of South Florida were studied for physical alterations that left sedimentary deposits ashore after the passage of Hurricane Donna as a Category 3 storm. There exist more than a dozen localities along the western shores and islands in the Gulf of California that qualify as coastal boulder deposits (CBDs) attributed to major storms that have struck since the start of the Pliocene Warm Period 4.5 million years ago and repeated during intervals of the Pleistocene and Holocene epochs until today [
25]. The CBD on the 12 m limestone terrace at Arroyo Blanco represents but one of these localities [
6]. Limited historical records indicate that 95 storms have struck Mexico’s Baja California peninsula since 1950 [
28]. Storm chasers risk their lives to follow tornados through tornado alley in the American Midwest, but few are foolish enough to camp out on an exposed rocky shore to observe the landfall of a hurricane. The challenge is to visit a known locality where such storms have struck previously but as soon as possible after the passage of a given storm. The 12 m limestone terrace at Arroyo Blanco on the east shore of Isla del Carmen is an ideal locality for this sort of surveillance. The locality (
Figure 1b) is far enough from populated areas that the terrace CBD remains undisturbed by human contact, but close enough to allow for periodic checks. What can be deduced about the influence of Hurricane Kay is that it did not have sufficient power to move the largest limestone slabs that were previously interpreted to have been displaced by earlier storms. However, smaller pieces of limestone with a distinct coloration were found to be scattered across the terrace. Examples are illustrated by two white chunks of stone that prop up the sea fan in
Figure 4a. The coloration matches no bedding surface elsewhere on the terrace and is assumed to have been eroded from a stratum in the subtidal zone.
Isla del Carmen is protected as part of the Loreto Bay National Marine Park, which also includes four other islands. These have no permanent human populations but are visited by boaters and kayakers mostly on day excursions. Although there is no substantial infrastructure that is vulnerable to damage by hurricanes, an ongoing program for monitoring changes in coastal geomorphology is desirable from a scientific point of view. In other parts of the world where large human populations live close to the coast, such a monitoring program is even more important. The essential groundwork for such environmental monitoring is already in place in the Philippines and mainland China, where typhoons that impact shorelines do significant damage to infrastructure [
10,
11]. The same attention is needed for conservation zones and nature preserves.
6. Conclusions
Under the continuing impact of global warning, major storms of hurricane intensity are a growing problem that not only affects the coastal zone of Mexico’s Baja California peninsula but other areas around the world. Surveillance studies of the kind demonstrated herein are a useful way to monitor the effect and growing danger of big storms.
What can be concluded with certainty is that wind-driven waves powered by Hurricane Kay disturbed the seabed directly adjacent to the limestone terrace, detaching sea fans from a depth no less than 5 m and sending them up and over sea cliffs to be left on the terrace as biological detritus at elevations on average more than 7 m above mean sea level (see
Table 1).
Likewise, seawater also splashed onto the limestone terrace and accumulated as widespread pools at an average elevation 7 m above mean sea level. Exposure to seawater killed bushes, most commonly Gulf Star Violets (
Stenotis mucronate). Some were growing in place as much as 33 m inland from the outer edge of the limestone terrace (see
Table 2). The plants also show that relatively long intervals by human standards may transpire before the repetition of a major storm.
The survey also detected previously unrecognized evidence for large potholes eroded on the limestone terrace that could only have formed under the influence of major storms. Such potholes are more commonly found in riverbeds where flooding is a major influence but are not widely recognized on rocky shores.