The present study represents the first attempt to provide palynological data about the southern Gulf of Mexico distal continental margin during the Eocene–Miocene time interval. In addition to documenting the types of fossil palynomorphs preserved in these economically critical rock sequences, this study also provides important information about the accumulation and preservation of total sedimentary particulate organic matter along with their paleoenvironmental and paleoceanographic implications. This was accomplished through the analysis of cored sediments from the Deep Sea Drilling Project Site 94 north of the Yucatan Peninsula and Site 540 south of the Western Florida Escarpment. The present findings from the deep waters of southern Gulf of Mexico are expected to greatly assist hydrocarbon geoscientists in their exploration and development efforts, reducing investment risk and drilling uncertainty.
Two palynofacies assemblages were recognized based on particulate organic matter analysis: marine-dominated palynofacies A and terrestrially influenced palynofacies B. The organic compositions of these two palynofacies along with the recovered dinoflagellate cyst community suggest that while sustained oceanic depositional conditions were prevalent, occasional influxes of terrestrial organic material of variable magnitudes occurred. These short-term influxes were especially pronounced during the late Eocene and earliest Oligocene and appear to have happened in response to the cooling and slight global sea-level drop that occurred at ca. 38–33.5 Ma. The preserved terrestrial palynomorphs in the samples indicate coastal plain and estuarine vegetation sources in general, except during the Eocene-Oligocene transition, when anemophilous pollen specimens mostly derived from farther inland and montane sources were recovered.
The Gulf of Mexico is considered one of the most important hydrocarbon megaprovinces in the world, with giant oil and gas discoveries and relentless growth in exploration by both domestic and international operators (Galloway, 2009). Exploration and production activities have been under way for several decades, mostly focusing on the petroleum systems along the coastal gulf region, which are typically Cenozoic in age and comprise good reservoirs created by wave-dominated paleoshorelines, and prolific source rocks produced by deltaic fronts (Galloway, 2001). Recent exploration efforts, however, have shifted to the thick siliciclastic deposits in the deep gulf. According to the July 2016 data published by the Bureau of Safety and Environmental Enforcement (https://www.bsee.gov), hydrocarbon production from the offshore Gulf of Mexico represented 96.4% (oil) and 95.5% (gas) of the total U.S. outer continental shelf oil- and gas-producing regions (Alaska, Pacific, Gulf of Mexico). The data also show that there were 23 new producible leases in 2015 from the Gulf of Mexico deep-water (>1000 ft [305 m] of water depth) fields, highlighting their enormous emerging economic value.
The Gulf of Mexico oceanic basin dates back to the Late Triassic, when the separation of the North American plate from the South American and African plates forced the Yucatan continental block southward (Salvador, 1987). The tensional grabens generated by this movement were eventually filled with red beds and volcanics. Although marine embayments were created, major flooding of the basin did not occur until the Middle Jurassic (Salvador, 1987). During the Early to Middle Jurassic rifting, the first salt deposits in the region formed in these restricted embayments. The subsequent rifting events that occurred throughout the Jurassic and Cretaceous formed less-extensive evaporite deposits (Philger, 1981). Since the Jurassic, the Gulf of Mexico has undergone extensive carbonate and siliciclastic deposition. The sedimentary load has shifted both vertically downward and laterally into the gulf—the former generated salt displacements and diapirs, and the latter formed extensional growth faulting (Peel et al., 1995). As part of studies aimed at understanding the sedimentation of ocean basins, the Deep Sea Drilling Project (DSDP) drilled sites in the Gulf of Mexico during multiple cruises (legs) in the 1960s to 1980s. Site 94 and Site 540 from two of these legs, Leg 10 and Leg 77, respectively, are the focus of this palynological study (Fig. 1).
Microfossil studies have been at the forefront in providing paleoenvironmental and paleoceanographic information about oceanic basins along with paleoclimatic data on adjacent landmasses. While much microfossil research in the Gulf of Mexico has focused on foraminifera and calcareous nannofossils, there is limited published information on the palynology of the sediments. Palynology has proven to be a useful tool in reconstructing the geologic history (paleoenvironments, paleovegetation, paleoecology, paleoclimatic changes, sea-level changes, relative ages) of Proterozoic and Phanerozoic sequences worldwide (Tyson, 1995; Traverse 2007). Palynomorphs are some of the few markers that can be used to correlate terrestrial and marine rock sequences because they are not facies restricted. Majority of the palynological studies in the Gulf of Mexico region have been largely concentrated on the U.S. Gulf Coastal Plain and the Yucatan Peninsula, and they examined mainly terrestrial palynomorphs, such as pollen, spores, and fungal remains (e.g., Frederiksen, 1980; Lenoir and Hart, 1988; Oboh and Morris, 1994; Elsik and Yancey, 2000; Harrington, 2003; Jarzen and Dilcher, 2006; Ramírez-Arriaga et al., 2006; Jardine and Harrington, 2008). These studies typically revealed few details about marine palynomorphs (dinoflagellate cysts, acritarchs, foraminiferal test linings, scolecodonts, etc.). Dinoflagellates and acritarchs flourish in marine environments, where their cysts are commonly preserved in sediments alongside transported pollen, spores, and other terrestrial palynomorphs. Some dinoflagellates have specific habitat requirements that allow their resting cysts to be used in determining paleoenvironmental conditions. In addition to specific types of palynomorphs, particulate organic matter (kerogen) provides useful palynofacies data for inferring important paleoenvironmental parameters such as redox conditions and proximal to distal trends.
The objectives of this study, therefore, were to: (1) identify and document the palynomorphs and particulate organic matter preserved in the middle Eocene to middle Miocene core sections of the DSDP Sites 94 and 540; (2) use dinoflagellate cyst and palynofacies data to interpret the depositional environmental conditions in the Gulf of Mexico; and (3) infer the paleovegetation on the adjacent landmasses by evaluating the diversity and types of terrestrial palynomorphs preserved at these deep-water sites and comparing them against published data for Paleogene to early Neogene taxa from the U.S. Gulf Coastal Plain region, eastern Mexico, and northern Colombia. These results will provide the fundamental elements necessary for understanding the regional climate within the context of the Paleogene–early Neogene global climate change.
MATERIALS AND METHODS
Drilling of Site 94 on the Campeche Escarpment in the northern edge of the Yucatan shelf occurred in March 1970 at a water depth of 1793 m. While initially interpreted as being related to a fault scarp, the escarpment shows evidence of having started as a drowned Albian barrier reef that backfilled with sediment (Worzel et al., 1973). Logan et al. (1969) described the shelf as the submerged part of a limestone plateau that gently slopes from south to north and is bounded by continental slopes that plunge into the Gulf of Mexico and Caribbean Sea. The 660 m section of Lower Cretaceous to Upper Pleistocene sediments penetrated by the drill consisted of limestone and lime mud (Early Cretaceous), foraminiferal and nannofossil chalk (Paleocene–Eocene), and foraminiferal and nannofossil ooze (Oligocene–Pleistocene).
DSDP Site 540 was cored west of the Southern Florida Escarpment in January 1981. The 745.5 m section of middle Albian to Upper Pleistocene sediments drilled consisted mostly of limestone with shallow-water skeletal debris and alternations of light and dark bands (middle Albian–early Cenomanian), gravity-flow deposits of pebbly chalk and limestone with calcareous volcanic sandstone (early/middle Cenomanian?–Campanian/Maastrichtian), and nannofossil and foraminiferal ooze, chalk, and marly limestone with ash layers (late Paleocene–late Pleistocene; Buffler et al., 1984).
The 58 samples analyzed for this study were obtained from cores stored at the Integrated Ocean Drilling Program (IODP) Gulf Coast Repository at Texas A&M University in College Station, Texas. From the 40 cores and 79 cores retrieved at Sites 94 and 540, respectively, 15 samples were taken from cores 9–16 (294.5–420.31 m below seafloor [mbsf]; Fig. 2), and 43 samples were taken from cores 6–29 (43.7–261.95 mbsf; Fig. 3). The sediments were processed following the standard laboratory processing technique outlined in Traverse (2007). Carbonates and silicates in the sediments were digested using hydrochloric and hydrofluoric acids, respectively. Residues from this digestion were used for kerogen slides before being centrifuged in heavy liquid (ZnBr2) and screened through a 10 μm sieve. Additional slides were prepared from the sieved residues, and samples with significant organic residues were stained using safranin red to improve visibility of dinoflagellate cysts. The slides were scanned for palynomorphs under a transmitted light microscope. Three-hundred palynomorph specimens were counted in productive slides, or all specimens if the slide population was below 300. Counting data can be found in the manuscript supplementary files.1 For palynofacies analysis, kerogen particles were classified into the following categories: terrestrial palynomorphs, marine palynomorphs, structured phytoclasts, degraded phytoclasts, opaque phytoclasts, and amorphous marine organic matter (see definitions in Oboh-Ikuenobe et al., 2005; Zobaa et al., 2011, 2015; El Beialy et al., 2016). Nikon polarizing microscopes and a Nikon Q-Imaging MicroPublisher 3.3 real-time viewing (RTV) digital camera were used in this study. Selected palynomorph and kerogen specimens are illustrated in Figures 4–6. All slides are currently housed in the palynological collection at Missouri University of Science and Technology.
Among the abundant dinoflagellate cyst taxa identified from this section, Operculodinium spp. comprised up to 64.8% of the total palynomorphs counted (Fig. 7). Identified species include Operculodinium centrocarpum, Operculodinium janduchenei, and Operculodinium longispinigerum. Operculodinium spp. reached their highest abundance during the early Miocene, but these species were also notably high during the latest early Oligocene–late Oligocene. There was a marked drop in their abundance at the late Oligocene–early Miocene boundary that was accompanied by an increased number of Spiniferites spp. In spite of their near absence in the late Eocene and earliest early Oligocene, Spiniferites spp. were the second most abundant dinoflagellate cysts in this section, comprising up to 19% of the total palynomorphs counted. Other recorded dinoflagellate cysts included Homotryblium spp. and Lingulodinium sp., which were generally rare (<3%). The acritarch genera Pterospermella and Micrhystridium constituted up to 25.4% of the total palynomorph assemblage and were particularly pronounced during the latest early Oligocene–late Oligocene.
Freshwater algae, embryophytic spores, and pollen of Pinaceae, Juglandaceae, Chenopodiaceae, Arecaceae (Palmae), and Cupressaceae were the main terrestrial palynomorphs recovered. Pollen of Pinaceae and Juglandaceae (Caryapollenites/Momipites) were very high in abundance (up to 73% and 46.4%, respectively) during the late Eocene–early Oligocene transition but decreased dramatically afterward. Palm pollen (Arecipites/Liliacidites) was common (46.7%) during the late Eocene, was not recovered throughout most of the early Oligocene, and occurred in low percentages (2%–11.4%) during the late Oligocene–early Miocene period. Cupressaceae pollen (mostly Cupressacites hiatipites) species were consistently present in all the samples, constituting ∼5%–20% of the total palynomorphs counted. Embryophytic spores were dominant late Eocene elements (up to 46.2%) but were not dominant in subsequent intervals. Except for a brief absence at the Eocene-Oligocene boundary, freshwater algae were consistently recovered and reached their highest abundance (36%) during the early Oligocene.
The paucity of palynomorphs in the Site 540 samples made it almost impossible to achieve statistical significance (i.e., adequate count; see Holt et al., 2011), resulting in the application of the presence/absence technique for data collection and interpretation. The early Oligocene portion of the core section was barren of both terrestrial and marine palynomorphs. Additionally, the late Eocene interval was barren of marine palynomorphs except for a single acritarch record. Among the identified dinoflagellate cysts, the deep-marine genus Impagidinium maintained continuous presence except during the late Eocene and middle Miocene intervals (Fig. 8). The late Oligocene interval preserved a moderately diverse assemblage of marine microplankton, which included Operculodinium spp., Spiniferites spp., Lingulodinium spp., Cleistosphaeridium sp., Homotryblium sp., and species of the acritarch genus Micrhystridium. As recorded in Site 94, the major terrestrial palynomorph components were freshwater algae, embryophytic spores, and pollen grains of Pinaceae, Cupressaceae, Arecaceae (Palmae), and Chenopodiaceae. However, Site 540 differed from Site 94 in preserving frequent occurrence of Poaceae (grass) pollen and the absence of Juglandaceae pollen, except for one record from the top of late Oligocene interval.
Particulate Organic Matter
Kerogen analyses for Sites 94 and 540 clearly indicate consistent dominance of amorphous marine organic matter (AMOM) over other particulate organic constituents (Fig. 9). This consistency was briefly interrupted by significant organic matter contribution from terrestrial sources during late Eocene and early Oligocene times. Two palynofacies assemblages were identified, namely, palynofacies A (marine dominated) and palynofacies B (terrestrially influenced; see also Zobaa and Oboh-Ikuenobe, 2009; Zobaa et al., 2009; Zobaa, 2011; Barron et al., 2013). At Site 94, palynofacies A consisted of a great amount of AMOM (up to 90% of the total kerogen) in addition to minor amounts of structured and degraded phytoclasts (4.5%–35.5%). Terrestrial palynomorphs and opaque phytoclasts were insignificant (0%–1.5%) in this assemblage. Palynofacies A was the dominant assemblage throughout sampled core sections at the Site 94, except at 420.31 mbsf and 407.19 mbsf (late Eocene), and 379 mbsf (early Oligocene), where palynofacies B was the prevailing assemblage with up to 99.5% terrestrial components (structured, degraded, and opaque phytoclasts). Although dominated by terrestrial organic components, palynofacies B also contained minor amounts of marine constituents (0.5%–29.5%).
Palynofacies data for Site 540 revealed an abundance of AMOM throughout the studied interval (Fig. 9), suggesting the dominance of marine influence (palynofacies A). In all but two samples (233.7 mbsf and 43.7 mbsf), palynofacies A contained greater than 73% AMOM of the total kerogen. Terrestrial components accounted for 10.3%–26%, with structured phytoclasts being the most common. Opaque phytoclasts were recorded but were less than 6.3%. Marine palynomorphs (almost solely dinoflagellate cysts) did not exceed 4.3%. Palynofacies B was represented by only one sample (233.7 mbsf, late Eocene), which was overwhelmingly composed of terrestrial structured phytoclasts (>69%), in addition to marine palynomorphs and AMOM (26%) and minor opaque phytoclasts and terrestrial palynomorphs. The sample at 43.7 mbsf (middle Miocene) appears to be transitional between palynofacies A and palynofacies B (Fig. 9).
Marine Depositional Environments
Knowledge about present-day dinoflagellates can provide information about past environmental conditions and distance from shore. Several dinoflagellate genera have preferred optimal habitats, and their distribution is controlled by upwelling, salinity, phosphate and nitrate concentrations, temperature, and water depth, among other factors (Marret and Zonneveld, 2003; Elshanawany et al., 2010). Diagnostic dinoflagellate cysts recovered in this study included species of Nematosphaeropsis, Impagidinium, and Spiniferites, in addition to Lingulodinium machaerophorum and Operculodinium centrocarpum. Information about the current habitats of these taxa was used to infer the paleoenvironmental conditions during the middle Eocene to middle Miocene (Table 1). While Operculodinium spp. are generally located in inner neritic to oceanic waters, their preferred habitat changes with latitude (Wall et al., 1977). Modern-day O. centrocarpum often lives near the surface, where evaporation raises the salinity of the water (Elshanawany et al., 2010). Spiniferites and Lingulodinium machaerophorum are more frequently found in neritic environments, but Spiniferites tolerates oceanic conditions (Limoges et al., 2013). Lingulodinium machaerophorum has strong preference for marine environments near river mouths today (Elshanawany et al., 2010).
At Site 94, the near-consistent occurrence of Impagidinium and intermittent recovery of Nematosphaeropsis throughout the studied section generally indicate deep-marine conditions (Udeze and Oboh-Ikuenobe, 2005; Elshanawany et al., 2010; Limoges et al., 2013). This observation is supported by the overall dominance of AMOM (palynofacies A), which are known to be the product of oceanic algal degradation under anoxic conditions (Oboh-Ikuenobe and de Villiers, 2003; Zobaa et al., 2011). The lithologies of the samples (foraminiferal and nannofossil ooze/chalk) further support this interpretation (Fig. 2). Although oceanic conditions remained more or less steady throughout deposition, the nature of organic matter contribution to the environment varied, probably in response to changes on the adjacent landmasses as well as global climatic and sea-level fluctuations. For instance, palynofacies B (Fig. 9) represents brief episodes during the late Eocene and early Oligocene characterized by strong input of terrestrially derived organic components into the marine environment. This terrestrial influence was probably the result of a short-term increase in continental runoff, in conjunction with the documented climatic cooling and global decrease in sea level (Haq et al., 1987; Zachos et al., 2001; Miller et al., 1998). Increased runoff is clearly seen in the palynomorph assemblage at depth 420.31 mbsf (Fig. 7), where abundant hygrophilous elements (freshwater algae and embryophytic spores) along with coastal/estuarine taxa such as Chenopodipollis and Cupressacites were all transported to the deep sea. It also appears that wind played an important role in transporting terrestrial palynomorphs to the deep sea, especially during times of reduced continental precipitation. This is particularly notable at the Eocene-Oligocene boundary, when the percentages of the anemophilous pollen (Pinaceae and Caryapollenites/Momipites) dramatically increased as freshwater algae and embryophytic spores almost disappeared. The late Oligocene and early Miocene sediments record an increase in the amount of marine palynomorphs (acritarchs, Spiniferites, and Operculodinium) over nonmarine taxa, reflecting much lower terrestrially influenced oceanic conditions.
Very high abundance of AMOM (palynofacies A) also dominated the Site 540 section, except for two late Eocene and middle Miocene samples (Fig. 9). This observation and the common occurrence of the deep-marine dinoflagellate cysts Impagidinium and Spiniferites argue for deep oceanic anoxic conditions. This interpretation is in agreement with the lithologies of the samples, which are predominantly nannofossil and foraminiferal ooze, chalk, and marly limestone (Fig. 3). It is evident, however, that under such deep-marine conditions, transportation of terrestrial organic matter from adjacent landmasses continued to take place, but in low percentages (average 18.3% of total kerogen). This also explains the presence, although very rare, of shallow-marine-indicative dinoflagellate cyst genera such as Operculodinium, Lingulodinium, and Homotryblium in the palynomorph assemblage (Fig. 8). During the latest Eocene (sample 233.7 mbsf), palynofacies B was highly prominent as accumulation of large amounts of terrestrial-structured phytoclasts occurred. This also appears to have been related to the aforementioned global cooling and slight sea-level drop noted for Site 94. Moreover, the noticeable age similarity between sample 407.19 mbsf at Site 94 and sample 233.7 mbsf at Site 540 (latest Eocene) and the similar kerogen composition (palynofacies B; Fig. 9) suggest strong correlation and warrant their use as chronostratigraphic marker horizons.
Information about the vegetation and/or depositional environment on the landmasses adjacent to the Gulf of Mexico was derived by evaluating the terrestrial palynomorphs preserved in the cores from Sites 94 and 540 and comparing them against previously published palynomorph records from 10 coastal localities in the United States, eastern Mexico, and northern Colombia (Fig. 10). Only a small fraction of the palynomorphs identified in these previous studies was found in the core samples from Sites 94 and 540. These spores and pollen were generally small in size (20–35 μm), were thin walled, and had reduced or no ornamentation. In spite of their low diversity and numbers, they provided some useful paleovegetation information (Table 1).
The preserved pollen in the middle to late Eocene samples at both sites included specimens of Chenopodipollis, Cupressacites, and Arecipites/Liliacidites (Palmae), suggesting coastal plain and estuarine vegetation sources (Frederiksen, 1985). These vegetation sources appear to have been progressively maintained during the Oligocene and Miocene, as indicated by the continued occurrence of these coastal/estuarine pollen species. The Eocene-Oligocene transition, however, recorded a conspicuous increase in the wind-pollinated upland/montane species of Pinaceae and Juglandaceae (Fig. 7), signaling long-distance, wind-driven transportation from inland mountainous areas. Pinaceous pollen species, in particular, tend to be selectively overrepresented in distal settings because of their outstanding hydrodynamic and eolian dispersion properties (Heusser and Balsam, 1977; de Vernal and Hillaire-Marcel, 2008).
Continental versus Marine Palynological Records
The diversity of spores and pollen in the 10 previously published studies in northern Colombia and the U.S. and Mexican Gulf Coasts (Fig. 10) far exceeded that preserved in the drill cores from Sites 94 and 540. Spores and pollen species common to the 10 sites include, for example, species of Arecipites/Liliacidites, Bombacacidites, Carya/Caryapollenites, Cupressacites, Deltoidospora, Ephedra, Momipites, Platycarya/Platycaryapollenites, Quercoidites, Ulmipollenites, and various bisaccate pollen that include Pinaceae (Frederiksen, 1985, 1998; Lenoir and Hart, 1988; Elsik and Yancey, 2000; Harrington, 2003, 2008; Yancey et al., 2003; Jarzen and Dilcher, 2006; Ramírez-Arriaga et al., 2006; Jardine and Harrington, 2008). Among these taxa, Arecipites/Liliacidites, Caryapollenites, Cupressacites, Momipites, Chenopodipollis, Quercoidites/Cupuliferoidaepollenites, and Pinaceae pollen were recovered in the core samples from Sites 94 and 540. A few dinoflagellate cysts, such as Hystrichokolpoma, Impagidinium, and Operculodinium, were recovered at the 10 coastal locations.
Frederiksen (1985), Tyson (1995), and Traverse (2007) noted that spore and pollen recovery generally decreased with increased distance between depositional sites and parent plants. These authors also noted that the wind-pollinated plants produced the greatest abundance of pollen, and such wind-transported pollen traveled the greatest distances. Those that do reach the more distal regions of the basin tend to have reduced or no ornamentation. Smaller pollen and spores are more common, but buoyant larger specimens can travel to offshore locations as well (Frederiksen, 1985). Core samples from Sites 94 and 540 were characterized by low spore and pollen diversity and higher proportion of wind-transported pollen. Other than pinaceous pollen grains, most recorded spores and pollen specimens were in the 20–35 μm size range. This observation confirms the offshore locations of Sites 94 and 540 during the middle Eocene to middle Miocene, and reasons for the disparity in the diversity and abundances of palynomorphs preserved there in comparison with the 10 aforementioned coastal studies. The higher abundances of terrestrial phytoclasts that characterize palynofacies B are the result of brief periods of increased input from land.
Our palynomorph and particulate organic matter investigation of core samples from DSDP Sites 94 and 540 provides valuable information about the nature and composition of preserved organic matter assemblages during the middle Eocene to middle Miocene interval in the southern Gulf of Mexico distal continental margin. Except for a few samples, amorphous marine organic matter (AMOM) was consistently recorded in very high abundances, which, together with the presence of deep-marine dinoflagellate cyst taxa (e.g., Impagidinium, Nematosphaeropsis, and Spiniferites), reflects the prevalence of deep oceanic conditions. During the late Eocene and early Oligocene, however, there were short-term episodes of strong terrestrial influence when ample amounts of structured phytoclasts were transported to the deep ocean setting. Such events appear to have occurred in response to the cooling and slight global sea-level drop at ca. 38–33.5 Ma.
A comparison of the terrestrial palynomorphs recorded in 10 published studies from the U.S. and Mexican Gulf Coasts and northern Colombia with those identified in the present study indicates that only a small fraction of the palynomorphs from the adjacent landmasses was preserved at Sites 94 and 540. These spores and pollen species were mostly wind transported, were thin walled, and had reduced or no ornamentation. Coastal/estuarine wetlands consistently supplied terrestrial palynomorphs throughout the middle Eocene to middle Miocene, although deposition at the Eocene-Oligocene boundary experienced a noticeable increase in the wind-transported upland/montane species.
We wish to thank the staff at Integrated Ocean Drilling Program (IODP) repository at Texas A&M University for providing the samples used in this research. The Josephine Husbands Radcliffe Fellowship is gratefully acknowledged for partially funding this study. Reviews by Elia Ramírez-Arriaga and an anonymous reviewer are much appreciated. Deep gratitude is expressed to Brad Singer, science editor, and Brian Pratt, associate editor, for their comments and editorial handling.
- Received 25 April 2016.
- Revision received 5 August 2016.
- Accepted 29 August 2016.
- © 2016 Geological Society of America