3.1 Seawater ingress into the Campos Basin
The Macabu Fm. in the Campos Basin is dominated by carbonate successions, which developed in an alkaline lacustrine setting under an arid climate 40, 41, 42. The overlying Albian aged Macaé Group in the Campos Basin clearly records sedimentation in a marine environment, as evidenced by foraminiferal biozonation and carbonate microfacies 43. Contrastingly, the Retiro Fm. is typified by evaporites, which represents the transitional phase between successions in the Macabu Formation and the Albian-aged Macaé Group, and may record the initial marine incursions in the Campos Basin.
Both carbonate clumped isotope (Δ47) values and the stereochemical configurations of biomarkers have reveal a moderate thermal history of the study area (SI). A wide array of lipid biomarkers, including n-alkanes, acyclic isoprenoids, terpanes, steranes, hopanes and carotenoids, were present in both free and desulfurized fractions (Fig. S5) from the Fragata core samples (Fig. 1). Of particular significance among the sterane biomarkers is the detection of 24-n-propylcholestane (24-npc) in both free non-polar (free-NP) and a second set of non-polar fractions generated by Raney Ni desulfurization (RN-NP) (Fig. 2). As a geological derivative of 24-n-propylidenecholesterol 24, 25, 24-npc is the major sterane diagnostic for marine chrysophyte/pelagophyte algae 44, 45. Accordingly, 24-npc is ubiquitously detected in marine environments and has been widely adopted as a diagnostic marker of marine or marine-influenced strata in the geological record 26, 27, 28, 29, 46. However, the detection of 24-npc may be confounded when in low relative abundance due to an interference signal from 4α-methyl-24-ethylcholestane (4α-methylstigmastane, 4-Me24Et) 21 that partially co-elutes on most GC liquid phases. Strict criteria are followed for the rigorous identification of 24-npc in the Fragata samples and include the relative signal intensities and the small but reproducible retention time difference between 24-npc and 4α-methyl-24-ethylcholestane 27. Ratios of 24-npc and 4-Me24Et (denoted as 24-npc ratio) are elevated in samples shallower than 5024.3 m in the Fragata core (26.7% ± 8.8%, 1σ, Table S2). This allows positive identification of 24-npc, since the 24-npc/4-Me24Et ratios in seawater-influenced settings are expected to exceed 10% 21, 28. Additionally, a reproducible offset of ~ 0.1 mins in the chromatographic peaks in the 414 → 217 and 414 → 231 Da transitions on GC-QQQ-MS (Fig. 2) further verified the marine signature of 24-npc at the top of the Fragata core. Significantly, 24-npc peaks are also recognized in the desulfurized RN-NP fractions (Fig. 2) with elevated signal intensities. Above 5024.3 m the 24-npc ratios rise to 73.0% (51.8% on average, Table S2). Accordingly, the occurrence of 24-npc in both free and sulfurized fractions, restricted to the entire interval above 5024.3 m in Fragata core, and upwards from the first appearance of anhydrite at the base of the Retiro Fm. indicates that marine waters had intruded into Campos Basin during the late Aptian (< 116 Ma).
Notable variations of 87Sr/86Sr were captured in the interval above 5024.3 m in the Fragata core (Fig. 3). The findings delineate distinct groups characterized by ratios falling within the ranges of 0.712–0.714 and 0.709–0.710. The initial interval, previously identified in several studies 40, 47, 48, is attributed to distinctly continental sources, serving as a significant geochemical indicator supporting the interpretation of the carbonate succession as having a lacustrine origin in the Campos and Santos basins. Despite the limited understanding of the paleo-tectonic context, the notable variability in the strontium isotope signal indicates a depositional system influenced by a range of sources, including continental cratonic rocks and contemporaneous basaltic volcanism as potential contributors 37, 49. The second interval, however, exhibits notably reduced radiogenic Sr values, predominantly around 0.709, indicating a distinct decrease in radiogenic sources entering the basin. Influx of marine waters would be reasonable, given that the anticipated average 87Sr/86Sr ratio for seawater during the Aptian period is approximately 0.707 50, with a projected rise in radiogenic Sr attributed to the evolution of the Paraná-Etendeka Large Igneous Province, as posited elsewhere 51. A mix between internally-drained solutes and marine influx would produce a hybrid brine with a new 87Sr/86Sr ratio under hydrological equilibrium. Considering fluctuations in water levels within a shallow environment, with a water/Sr mass balance sensitive to source variations, it may be possible to explain the variation in radiogenic strontium within the Macabu Formation due to changes in sources that are influenced by tectonic activity. The excursions of 87Sr/86Sr ratio are indicative of sources related to mafic volcanic rocks and/or marine incursions. While other strontium excursions are observed in the lower intervals of the pre-salt carbonates47, 52, a significant shift at the top of the succession, near the contact with the Retiro Formation, suggests this point as the onset of predominant marine influx.
Paleontological evidence has implied that seawater infiltrated from the NE Brazilian marginal basins, including the Sao Luís, Parnaíba and Araripe basins during the transition of Aptian–Albian; Furthermore, a Tethyan ingression is suggested by the presence of several fossil taxa 8, 13, 14, 15. However, there is a lack of unequivocal marine evidence in the SE Brazilian marginal basins (e.g., Campos and Santos Basins). Accordingly, a series of southward marine transgressions from the equatorial Tethys Ocean was hypothesized 7, 8 divergent from the tectonic evidence of a marine pathway extending from south to north on the basis of sea-floor spreading patterns and geodynamic reconstructions 16, 17, 18, 19, recently acquired geochronological data 37, 49 and from evidence gathered from the conjugate margins of West Africa 9.
3.2 A switch to a marine-influenced ecosystem
Numerous examples have illustrated how seawater entry into lacustrine ecosystems alters their hydrography, water chemistry and biology. Ingressions can result from subtle changes in relative sea level on glacial-interglacial timescales (e.g. 53) or from tectonism on geological timescales 27, 28, 29, 53. The salinity and depth of a water body, together with basin shape, will influence the tendency towards density stratification. Relative abundances of methyltrimethyltridecylchromans (MTTCs) isomers (e.g., methylated MTTCs) have been proposed to be a reliable indicator for paleosalinity in aquatic surface-layers 54, 55, especially recording the periodic incursions of marine waters and subsequent evaporation 56. Here, the Fragata core records enhanced MTTCs ratios (0.02 vs. 0.16 on average, with the highest value to 0.42, Table S2, Fig. 3), pointing to a rapid elevation in salinity above 5025.12 m. Subsequently, the elevated abundance of gammacerane (denoted by the gammacerane index, GI) is additionally indicative of enhanced water column stratification, given that its precursor, tetrahymanol, is a signal for bacterivorous ciliates living at redox transitions 57, 58. Stratification reduces mixing and thus promotes oxygen-depletion in deeper waters, as evidenced by concomitant reduction in the pristane/phytane (Pr/Ph) ratio and enhancement of C35 Homohopane Index (C35 HHI) at the top of the section (Fig. 3). Pr/Ph is regarded as a redox-sensitive proxy due to the preferential formation of phytane over pristane derived from the phytol side-chains of chlorophylls under anoxic conditions 59. Although additional factors including salinity, organic matter sources, and diagenetic process may affect Pr/Ph 60, 61 values below 1 are considered diagnostic for pervasive anoxia. Similarly, the C35 HHI is a robust proxy for tracing anoxic conditions 21, considering that the C5 side-chains of its precursor compounds, C35 bacteriohopanepolyols, are best preserved under the anoxic and H2S-rich conditions that follow from enhancement of sulfate levels from seawater incursions 62, 63. Together, the evolution of multiple biomarker proxies documents a hydrographically dynamic system, capturing the establishment of enhanced anoxic/euxinic, saline and stratified conditions in the Retiro Formation.
Given the enhancement of anoxia induced by the marine incursion, reductive sulfurization of organic matter under euxinic conditions 33 becomes an issue that may complicate or inhibit the identification of some biological precursor lipids 64. In fact, here, the patterns of n-alkanes, and of sterane and hopane stereoisomers released by the RN treatment (Fig. S5–S6), illustrate how specific biomarkers have been sequestered into the S-bound macromolecular component of sedimentary organic matter. High abundances of phytane and C35 homohopanes released during desulfurization (Fig. 4-D) further confirm their preferential preservation under anoxic and sulfidic conditions 59, 63. Despite striking differences in the patterns of the free-NP and RN-NP fractions, however, the key biomarker proxies all move in a consistent direction concomitant with the marine incursion. Both free and sulfurized lipids also reveal a dynamic system: Here, the ingress of seawater strengthened stratification within this sector of the Campos Basin, fostering the development of water column anoxia, as evidenced by elevated values of GI, C35 HHI and MTTC (Fig. 5). Accordingly, hydrographic changes induced by seawater ingress have prompted ecosystem re-structuring as recorded in its biochemostratigraphy. A striking and progressive elevation in sterane/hopane ratios (S/H) (up to 10.6, Table S2), a proxy based on the diagenetic products of sterols and hopanoids diagnostic for eukaryotes and bacteria respectively 21, 65, suggests this environmental shift appears to have favored photosynthetic algae, allowing their proliferation at the expense of bacteria. Consistent with transgressive records in other basins 23, 27, the elevated S/H document the ingress of seawater providing algae with an advantage over the bacterial community whether by addition of nutrients or other factors associated with the water chemistry. Among all steroid derivatives, stigmastane (C29 sterane) becomes dominant over cholestane, its C27 counterpart, after the marine incursion (Fig. 4-C; Fig. 5). Given their mostly biological source from Chlorophyceae and Rhodophyceae, respectively 66, 67, the enhanced stigmastane represents the likely prevalence of green algae over other taxa in the immediate aftermath of the transition.
3.3 A progressive deepening of the chemocline with marine ingress
Despite the detection of the array of carotenoids in the free biomarker fractions (Table S2), diverse inventories of aliphatic and aromatic carotenoids are released upon desulfurization of the polar fractions (Fig. 4-A, B). In the free-NP fractions, the predominant compounds in all samples are β-carotane and isorenieratane, together with minor β-isorenieratane and C38 carotenoids. However, in the RN-NP fractions, relatively high abundances of isorenieratane, chlorobactane and okenane that have been sequestered into macromolecules by sulfurization supports the notion that they represent indigenous signals for phototrophic sulfur bacteria. As strict anaerobes, these phototrophic sulfur bacteria, comprising green sulfur bacteria (i.e., GSB, Chlorobiaceae) and purple sulfur bacteria (i.e. PSB, Chromatiaceae), utilize sulfide and other reduced sulfur species as electron donors for photosynthesis 68. Because of their dual requirements for illumination and sulfide, the GSB and PSB are considered index species for photic zone euxinia (PZE) where the light penetrates a sulfide-containing water column 69. Thus, their biomarker lipids have been applied as PZE biosignatures in both modern environments and in paleoenvironmental reconstruction 64, 65, 70, 71, 72, 73. Being the most widely recorded aromatic carotenoid in marine sediments 71, 74, isorenieratane is the diagenetic product of isorenieratene that is derived from low-light-adapted brown strains of phototrophic green sulfur bacteria (GSB). These GSB are known to be well adapted to lower light intensities and therefore deeper waters (~ 100 m) 69, 75. In comparison, green-colored strains that produce chlorobactene, the biological precursor of chlorobactane, require higher light intensity and accordingly are concentrated at shallower (∼15–30 m) chemocline depths 76. In contrast, the okenone-synthesizing PSB can proliferate as plankton within a shallow chemocline (~ 20 m; 77), in benthic microbial mats, as aggregates with sulfate reducing bacteria in the water column 78 or, perhaps reflecting their slightly higher tolerance for oxygen, as observed in the ‘pink berry consortia’ that are found in tidally-influenced marginal marine environments 79.
In the present study, by comparing the sedimentary lipids of the free and S-bound fractions, we observe a novel switch in the assemblages of aromatic carotenoids upon initial marine ingress into the Campos Basin. At the base of the anhydrite-bearing layer that marks the onset of the incursion, a peak in PSB-derived okenane (up to 33.0%, Fig. 6, Table S2) implies extremely shallow PZE or microbial mat facies. Okenane then declines coincident with a peak in the abundance of GSB-derived chlorobactane (3.8%) and elevated isorenieratane. After this, isorenieratane becomes the dominant aromatic carotenoid identifying the brown-pigmented GSB as the predominant anoxygenic phototroph and a deepening chemocline. In addition, the composition of C38 aromatic carotenoids decrease markedly (6.4% vs. 18.2%, p < 0.001, Table S2). These C38 aromatic carotenoids are the diagenetic products of synechoxanthin which is a C40 aromatic carotenoid with dual carboxylic acid functionalities prevalent in non-marine or euryhaline cyanobacteria 22, 70, 80, 81. Thus, reductive sulfurization and decarboxylation affords the C38 diagenetic products together with C39 counterparts diagnostic for cyanobacteria that are prevalent while lacustrine conditions prevail. Their decline coincides with the increases in phototrophic sulfur bacteria that accompany the ingress of seawater. In other words, the marine incursions instigated an overturn of the photosynthetic community, including a proliferation of eukaryotic algae together with GSB concomitant with a decline in primary productivity by the synechoxanthin-producing cyanobacteria. In summary, both the less radiogenic 87Sr/86Sr ratio and 24-npc biomarker results indicate marine incursion associated with the deposition of the Retiro Formation, associated with a fundamental transformation in the biological community.