Benthic Foraminiferal Response to the Aptian−Albian Carbon Cycle Perturbation in the Atlantic Ocean

The Aptian−Albian boundary interval (AABI) is characterized by a prominent carbon cycle perturbation. Several pulses of increased organic carbon burial in marine sediments worldwide characterize the Oceanic Anoxic Event (OAE) 1b across the AABI (e.g., Herrle et al., 2004; Coccioni et al., 2014; Bottini & Erba, 2018). One of these pulses, the Kilian sub-event, defines the Aptian−Albian boundary, and is characterized by a negative stable carbon isotope excursion (δ¹³C_carb CIE) recorded in marine carbonates (Herrle et al., 2004; Kennedy et al., 2014, 2017). This negative CIE implies a massive input of light (¹²C) carbon into the ocean and atmosphere reservoirs. A possible source of relatively light carbon may have been carbon dioxide (CO₂) degassing from volcanism related to placement of the Kerguelen Plateau and/or widespread submarine volcanism (Sabatino et al., 2018; Matsumoto et al., 2020). Since osmium isotope records of marine sediments across the AABI depict varying signals, it is also possible that deposition of organic carbon-rich levels within OAE 1b was driven by a combination of volcanic CO₂ release and enhanced monsoonal activity (Matsumoto et al., 2022).

High-resolution studies of planktic foraminiferal assemblages across the AABI depicted a major faunal turnover, with virtually all large, heavily calcified and macroperforate Aptian taxa becoming extinct during the latest Aptian, and their replacement by smaller, thin-walled and microperforate taxa in the earliest Albian (Huber & Leckie, 2011; Petrizzo et al., 2012). Percentages of planktic foraminifera, relative to benthic ones, also dropped dramatically at the AABI, reaching nearly 0% during the earliest Albian in the Atlantic Ocean (Huber & Leckie, 2011). Concerning benthic foraminiferal assemblages, high-resolution records across the AABI in the North Atlantic suggested no turnover correlated with OAE 1b (Holbourn & Kuhnt, 2001), although a later revision of the planktic foraminiferal biostratigraphy in that sedimentary archive revealed a major unconformity at the Aptian−Albian boundary (Huber & Leckie, 2011). Nevertheless, the dominance of calcareous benthic foraminifera within OAE 1b black shales on Blake Plateau is puzzling, since older intervals of black shale deposition were usually dominated by agglutinated benthic species (Holbourn et al., 2001). This observation suggests that new benthic foraminiferal adaptive strategies may have evolved within the AABI. Since most of the benthic foraminiferal records across the AABI are of relatively low-resolution (e.g., Basov & Krasheninnikov, 1983; Holbourn and Kaminski, 1997; Kochhann et al., 2014; Giraldo-Gómez et al., 2022), it is crucial to explore high-resolution continuous sedimentary archives to test how benthic foraminiferal assemblages responded to the carbon cycle perturbation at the Aptian−Albian transition.

Here we present a high-resolution assessment of benthic foraminiferal distributions across the AABI at Deep Sea Drilling Project (DSDP) Site 511, located in the southern South Atlantic Ocean, and Ocean Drilling Program (ODP) Site 1049, located in the western subtropical North Atlantic Ocean (Fig. 1 ). We selected these sites due to their excellent carbonate preservation, and because they present a clear record of the planktic foraminiferal turnover across the Aptian−Albian boundary (Huber & Leckie, 2011). Our main goals are: (i) to evaluate whether benthic foraminiferal assemblages in the Atlantic Ocean were affected by the carbon cycle perturbation at the AABI, and (ii) to assess environmental factors that may have driven faunal reorganizations among benthic foraminiferal assemblages over the AABI.

Site 511 is located on the Falkland Plateau (51°00.28’S, 46°58.30’W, water depth of 2589 m). During the Aptian−Albian, it was situated at a paleolatitude of about 50°S (Fig. 1), and sediments were deposited at paleodepths between 100 and 400 m (Basov & Krashenninikov, 1983). We studied the interval between Core 511-56R (495.63 meters below seafloor – mbsf) and Core 511-49R (430.74 mbsf). This interval was assigned to lithologic unit 4, which is composed of claystones, nannofossil claystones, and muddy nannofossil chalks (Ludwig et al., 1983).

At Site 1049, we studied Hole 1049C, located on the Blake Plateau (Blake Nose; 30°8.5370’N, 76°6.7271’W, water depth of 2640.8 m). Site 1049 remained at a paleolatitude of about 30°N (Fig. 1), and Albian sediments were deposited at middle bathyal paleodepths (Kroon et al., 1998), corresponding to 600-1000 m water depth (Van Morkhoven et al., 1986). Sampling spans Core 1049C-13X (153.05 mbsf) to Core 1049C-11X (132.64 mbsf). This interval was assigned to lithological unit 4A, which is composed of clayey nannofossil chalk, nannofossil clay, clay and one black shale level within Core 1049C-12X (Kroon et al., 1998).

Stratigraphy for Sites 511 and 1049 follows the planktic foraminiferal biostratigraphy of Huber & Leckie (2011) for Site 1049 and Huber et al. (2018) for Site 511. We updated the Site 511 zonation, identifying the Microhedbergella renilaevis Zone, following the definition of Petrizzo et al. (2012) and Kennedy et al. (2014). At Site 511, the base of the M. renilaevis Zone occurs at 485.11 mbsf, defined by the first occurrence (FO) of M. renilaevis. At Site 1049, however, we could not identify the M. renilaevis Zone due to the unconformity occurring at the Aptian−Albian boundary (Huber & Leckie, 2011). The revised planktic foraminiferal biostratigraphy at Site 511 is presented in Supplementary Figure S1. We collected 35 samples from Site 511 and 23 samples from Hole 1049C, with sampling resolution varying between 4 m and 1 cm, depending on the proximity of samples to the Aptian−Albian boundary.

Samples were prepared following the procedure described in Huber & Leckie (2011): sediments were soaked into tap water for 24 hours, washed over 63 μm sieves, and residues were convection dried at 50°C. We picked between 250 and 400 benthic foraminiferal specimens from all >63 μm residues, whenever possible.

All benthic foraminifera were classified following taxonomic definitions of Loeblich & Tappan (1987), for generic identifications, and mostly Basov & Krasheninnikov (1983), Holbourn & Kaminski (1997), Holbourn & Kuhnt (2001), and the original descriptions in the Ellis & Messina database (https://www.micropress.org/ellis-messina.html) for identifications at the species level. We assigned each identified taxon to an epifaunal, epifaunal-infaunal (mobile) or infaunal microhabitat preference according to Koutsoukos & Hart (1990).

We calculated diversity indices [species richness (S) and Shannon-Weaver diversity (H)] and ran a hierarchical cluster analysis using the software Past 4.09 (Hammer et al., 2001). For the cluster analysis, we used a subset of species with average abundances >1% for each site; clustering was performed with the Ward's method algorithm using Euclidian distance as a similarity index. Cophenetic correlations were 0.88 and 0.85 for sites 511 and 1049, respectively, attesting the statistical significance of the clustering procedure. All datasets are provided in Supplementary Tables S1 and S2.

Considering broader distribution patterns, late Aptian−Albian benthic foraminiferal assemblages at Site 511 were diverse and, overall, well preserved (Figs. 2 , 3 , Appendix 1, Supplementary Table S1). Species richness and diversity increase, reaching S = 46 and H = 3.0, respectively, in the upper Aptian (Paraticinella rohri to Microhedbergella miniglobularis zones (492.72 50 to 486.27 mbsf). Across the Aptian−Albian boundary (Microhedbergella renilaevis Zone), richness and diversity decrease to ∼30 and 2.5, respectively, in tandem with the negative δ¹³C excursion assigned to the Kilian subevent (Fig. 4 ). Within the lower Albian, species richness and diversity decrease at the base of the M. rischi Zone (484.92 to 462.73 mbsf), to values as low as 19 and 1.5, respectively, and progressively recover above 462.73 mbsf, within the M. rischi and Ticinella raynaudi (= “T. yezoana”) zones (Fig. 4).

Relative abundances of agglutinated and benthic foraminifera and species interpreted as having preferred infaunal microhabitats (Appendix I) progressively increase within the uppermost P. rohri and M. miniglobularis zones (from values about 10% at 487.11 mbsf to 30% each at 485.36 mbsf), with peak values within the negative δ¹³C excursion of the Kilian subevent (Fig. 4). At this level, relative abundances of calcareous and inferred epifaunal species decrease from 90% to ∼70% and from 70% to ∼50%, respectively, (Fig. 4). Relative abundances of agglutinated and inferred infaunal benthic foraminifera decrease abruptly at the Aptian−Albian boundary, at the base of the M. renilaevis Zone, with relative abundances of calcareous and inferred epifaunal species progressively increasing between 486.16 and 484.45 mbsf (Fig. 4). Relative abundances of agglutinated and inferred infaunal species remain low from the early Albian M. rischi to T. raynaudi zones, averaging 12% and 8%, respectively, between 483.65 and 430.74 mbsf (Fig. 4). Relative abundances of calcareous benthic foraminifera increase by ∼10% from 483.65 to 462.73 mbsf, followed by a decrease of ∼10% to the top of the studied interval (430.74 mbsf; Fig. 4).

At the species level, we identified 86 benthic foraminiferal taxa at Site 511 (Fig. 2, 3; Supplementary Table S1). The most representative species, with average relative abundances of >1%, were clustered into two groups (Fig. 5 ), which present characteristic stratigraphic distributions. Group 1 is composed of weakly calcified and agglutinated taxa, such as Globulina lacrima, Patellina subcretacea, Ammodiscus cretaceus, Glomospira gordialis, Laevidentalina communis, Saracenaria triangularis, Lenticulina pulchella, Pyrulina spp. and Praedorothia ouachensis (Fig. 5). The summed relative abundances of species clustered in Group 1 were relatively high and declined progressively within the upper Aptian (Hedbergella trocoidea to Microhedbergella miniglobularis Zones), from 26% at 494.18 mbsf to 8% at 485.36 mbsf (Fig. 6 ). Group 1 abundances remain low (mostly <10%) within the lower Albian M. renilaevis and M. rischi zones, and only recover within the Ticinella raynaudi Zone, above 437.61 mbsf (Fig. 6). Group 2 is mostly composed of heavily calcified species assigned to the genera Berthelina, Gyroidinoides, Osangularia and Scheibnerova, except for the agglutinated species Gaudryina dividens (Fig. 5). Summed relative abundances of species assigned to Group 2 are low (averaging 65%) from the H. trocoidea to the P. rohri zones (495.63 to 486.6 mbsf), increase by ∼20% within the M. miniglobularis and M. renilaevis zones (486.27 to 485.16 mbsf), and remain high over most of the T. raynaudi Zone, declining only above 437.61 mbsf (Fig. 6).

Benthic foraminiferal assemblages at Site 1049 were also diverse and well preserved within the AABI (Figs. 2, 3; Appendix 1, Supplementary Table S2). The Aptian−Albian boundary at Site 1049 is characterized by an unconformity estimated as 0.84 m.y. in duration, evidenced by the absence of the M. renilaevis and M. miniglobularis zones and an age model that was constructed across the interval (Huber et al., 2011). Therefore, faunal parameters depict abrupt changes across the Aptian−Albian transition. Considering overall trends, species richness and diversity increase through the upper Aptian (between 153.05 and 145.29 mbsf), peaking (S = 44, H = 3.2) within the upper P. rohri Zone (Fig. 7 ). Both indices decrease abruptly in the lower M. rischi Zone, between 145.25 and 142.73, and recover progressively from the middle M. rischi Zone to the Ticinella madecassiana Zone (141.11 to 132.64 mbsf).

Relative abundances of agglutinated benthic foraminifera are higher, averaging 23%, in the upper Aptian (153.05 to 145.29 mbsf), and drop to an average value of 5% in the Albian (145.25 to 132.67 mbsf; Fig. 7). The opposite trend was depicted by relative abundances of calcareous benthic foraminifera, with a mean value of 77% in the Aptian (153.05 to 145.29 mbsf), increasing to an average value of 95% within the Albian (145.25 to 132.67 mbsf; Fig. 7). Relative abundances of foraminifera interpreted as having had infaunal microhabitat preferences (Appendix I) increase progressively through the upper Aptian (153.05 to 145.29 mbsf), fall markedly within the lowermost M. rischi Zone (145.25 mbsf to 144.32 mbsf), and recover upward in the section (Fig. 7). The drop in relative abundances of inferred infaunal species in the lowermost M. rischi Zone is coupled with a ∼25% increase in relative abundances of inferred epifaunal foraminifera between 145.25 and 144.32 mbsf (Fig. 7).

We identified 84 benthic foraminiferal species in the studied interval of Site 1049 (Figs. 2, 3; Supplementary Table S1). The most representative taxa were clustered into three groups (Fig. 5), and their total relative abundances are presented in Figure 8 . Group 1 is characterized solely by Osangularia schloenbachi (Fig. 5), which is virtually absent from upper Aptian assemblages (Globigerinelloides algerianus to Paraticinella rohri zones; 153.05 to 145.29 mbsf), but peaks in relative abundance (79%) within the lowermost M. rischi Zone between 145.25 to 141.11 mbsf (Fig. 8). From the middle M. rischi Zone to the T. madecassiana Zone, between 139.46 and 132.64 mbsf, Osangularia schloenbachi displays a mean relative abundance of 16%. Group 2 is mostly composed of weakly calcified and agglutinated taxa assigned to the genera Ammodiscus, Gaudryina, Globulina, Glomospira, Lenticulina, Pleurostomella, Saracenaria, Spirillina, Tritaxia, and Praebulimina. Summed relative abundances of group 2 average 50% within the upper Aptian Globigerinelloides algerianus to Paraticinella rohri zones (153.05 to 145.29 mbsf) and drop to an average value of 14% within the lower Albian M. rischi to T. madecassiana zones (145.25 to 132.64 mbsf; Fig. 8). Group 3 is dominated by heavily calcified taxa, such as Berthelina intermedia, Gyroidinoides infracretaceus, Schneibrova protindica and Valvulineria gracillima (Fig. 5). This group presents low summed abundances within the upper Aptian G. algerianus to P. rohri zones (153.05 to 146.51 mbsf), increases in abundance within the uppermost P. rohri Zone (averaging 55% between 145.99 and 145.1 mbsf), declines across the AABI unconformity and lowermost M. rischi Zone to an average value of 18% between 144.9 and 144.32 mbsf, and recovers to and average value of 50% between the M. rischi and T. madecassiana zones (142.73 to 132.64 mbsf; Fig. 8).

Benthic foraminiferal distributions at Sites 511 and 1049 do not depict major increases in extinction (last occurrences) nor speciation (first occurrences) rates across the Aptian−Albian boundary (Figs. 4, 7). Increased numbers of extinctions at the upper end of our records, and first occurrences at their bottom ends, are most likely an edge artifact, since no samples were analyzed higher up or lower down, respectively, in the stratigraphic successions of Sites 511 and 1049. Our high-resolution records thus support the notion that the faunal turnover observed among planktic foraminifera across the AABI was not coupled with a major turnover among benthic foraminifera, in agreement with the high-resolution census counts at Site 1049 (Holbourn & Kuhnt, 2001; Huber & Leckie, 2011). In fact, lower-resolution studies in the Atlantic (Basov & Krasheninnikov, 1983; Kochhann et al., 2014), Indian (Holbourn & Kaminski, 1997) and Pacific (Giraldo-Gómez et al., 2022) oceans corroborate this interpretation.

Considering that the sedimentary archive at Site 1049 presents an unconformity at the Aptian−Albian boundary (Huber & Leckie, 2011), faunal records at Site 511 are more reliable for depicting high-frequency changes in the structure of benthic foraminiferal assemblages close to the Aptian−Albian transition. Benthic foraminiferal richness and diversity dropped across the CIE at the Aptian−Albian boundary (Fig. 4). These patterns imply that benthic foraminiferal assemblages in the Atlantic Ocean were affected by processes related to carbon cycling during the AABI (see next section), although there was no major extinction event within the CIE at the Aptian−Albian boundary. Despite the occurrence of the Aptian−Albian unconformity at Site 1049, the records from that site also depict drops in species richness and diversity within the lowermost lower Albian M. rischi Zone (Fig. 7). Increased extinction rates within the M. rischi Zone (n = 8) at Site 1049 were likely related to the replacement of infaunal by epifaunal taxa (see discussion below), reflecting changes in local environmental conditions.

In summary, no major faunal turnover among deep water benthic foraminifera (from bathyal to abyssal paleodepths) occurred across the AABI, although the associated planktic foraminiferal turnover was the second most important event recorded in the planktic foraminiferal fossil record beside the Cretaceous-Paleogene (K-Pg) boundary event (e.g., Huber & Leckie, 2011; Petrizzo et al., 2012). This observation suggests that environmental disturbances were more severe in the upper water column during both events (e.g., Thomas, 1990; Culver, 2003; Alegret & Thomas, 2004, 2007, 2013), despite the fact that more studies are needed to characterize the drivers of environmental changes within the AABI.

As in Tethyan sections, the Aptian−Albian transition at Site 511 was characterized by >2‰ negative δ¹³C and δ¹⁸O excursions (Figs. 4, 6; Herrle et al., 2004; Kennedy et al., 2014, 2017; Huber et al., 2018). These negative excursions indicate the occurrence of a massive input of light carbon (¹²C) into the atmosphere-ocean system coupled with transient global warming. Although δ¹⁸O values could be diagenetically biased due to deep burial of Cretaceous sedimentary sequences, such as the 430−500 m burial depth at Site 511, independent sea surface temperature reconstructions based on calcareous nannofossil assemblages from the western Tethys indicate warm temperatures across the Kilian subevent (= Aptian−Albian Boundary; Bottini et al., 2015). Deep ocean records of mercury concentrations and osmium isotopes suggest that multiple submarine volcanic episodes, such as at the Kerguelen Plateau, occurred at the Aptian−Albian transition (Sabatino et al., 2018; Matsumoto et al., 2020), releasing substantial volumes of isotopically light CO₂ into the oceanic and atmospheric reservoirs.

Between 495.63 and 475.22 mbsf at Site 511, relatively high background total organic carbon (TOC) content and cyclic increases of the V/Al ratio correlate with the interval recording abundance peaks of infaunal foraminifera (coeval with high V/Al maxima) within the P. rohri to lower M. rischi zones (Fig. 4). High values of these geochemical tracers suggest that sediments were deposited under poorly oxygenated bottom waters, which were only favorable for infaunal foraminifera (Dummann et al., 2021). Modern infaunal species tend to dominate assemblages under increased organic carbon burial, since they are able to migrate upward in the sediment column, as bottom and pore water oxygenation decrease, replacing less tolerant epifaunal taxa (e.g., Jorissen et al., 1995; Jorissen et al., 2007). Our assignments of microhabitat preferences are based on correlations of test morphology with microhabitat, which are overall applicable in modern environments despite some complications (Jorissen, 2003), and are at least partly applicable to Cretaceous taxa (Koutsoukos & Hart, 1990). In fact, the highest TOC contents in the studied interval of Site 511 occur below the Aptian-Albian transition, within the upper P. rohri zone (= upper NC7 calcareous nannofossil zone; Fig. 4). Benthic foraminiferal assemblages within this TOC-rich interval were extremely impoverished, yielding only 27 and 129 specimens at 494.18 and 492.72 mbsf, respectively. We assume that poorly oxygenated bottom waters limited benthic foraminiferal occurrences within the P. rohri/NC7 zones at Site 511, hampering reliable interpretations of relative abundance changes of morphogroups and species over this interval. Particularly across the AABI, our high-resolution records at Site 511 depict a ∼20% increase in the relative abundances of infaunal benthic foraminifera (e.g., Group 1) between 487.34 and 485.36 mbsf. This transient increase of infaunal species abundances occurred in tandem with the negative δ¹³C and δ¹⁸O excursions at the Aptian−Albian boundary (Figs. 4, 6), suggesting increased organic carbon burial and decreased bottom water oxygenation conditions.

Regardless of the unconformity at the Aptian−Albian boundary at Site 1049, which hampers characterizing the Kilian sub-event CIE, abundances of infaunal species (e.g., Group 2) increase up-section within the upper Aptian, from the Globigerinelloides algerianus to the P. rohri zones (Figs. 7, 8). Increased abundances of infaunal taxa at Sites 511 and 1049 suggest that enhanced organic carbon burial and decreased bottom water oxygenation were widespread features in the Atlantic Ocean during the latest Aptian. Comparable intervals of reduced bottom water ventilation during the latest Aptian were described for western Tethys sections, such as in the Vocontian Basin (e.g., Herrle et al., 2003; Friedrich et al., 2003), and at the Poggio le Guaine core in Italy (Ferraro et al., 2020). Overall, our new, and compiled, proxy records suggest that the latest Aptian global warming was characterized by reduced ocean floor ventilation, probably linked with more sluggish circulation patterns.

During the early Albian at Sites 511 and 1049, relative abundances of epifaunal benthic foraminiferal species increased (Figs. 4−8). Modern epifaunal taxa tend to thrive under reduced organic carbon flux and enhanced bottom water oxygenation (e.g., Jorissen et al., 1995; Jorissen et al., 2007). Decreased richness and diversity within the M. rischi Zone were likely driven by the dominance of heavily-calcified epifaunal taxa at Sites 511 and 1049 (Figs. 4, 7). In fact, at Site 1049, relative abundances of epifaunal taxa increased only briefly within the lowermost M. rischi Zone (144.90 to 144.32 mbsf), followed by the highest relative abundance of infaunal species at 142.73 mbsf, and a decreasing-upward trend throughout the M. rischi and T. madecassiana Zones (Fig. 7). Earliest Albian epifaunal assemblages at Site 1049 were also dominated by Osangularia schloenbachi (Group 4 in Fig. 8), which was likely an opportunistic species that tolerated poorly oxygenated bottom waters (e.g., Erbacher et al., 1999; Friedrich et al., 2005; Friedrich, 2010). It is thus likely that better deep water ventilation prevailed at the high latitude Site 511 than at Site 1049 during the early Albian.

There is no evidence of carbonate dissolution during the Aptian at Sites 511 and 1049, as attested by the relatively high carbonate content at Site 511 (Fig. 4; Dummann et al., 2021), and the consistent occurrence of well-preserved calcareous microfossils (Figs. 2, 3). Relatively shallow paleodepths at the studied sites likely ensured good carbonate preservation: Site 1049 probably reached middle bathyal depths, between 600 and 1000 m water depth, during the Albian (Kroon et al., 1998), whereas Site 511 sediments were deposited at 100 to 400 m water depth (Basov & Krashenninikov, 1983). Nevertheless, we speculate that changes in the carbonate chemistry of sea and/or pore waters affected benthic foraminiferal abundances across the AABI at the studied sites.

The latest Aptian interval of decreased bottom water oxygenation and enhanced organic carbon burial (G. algerianus to M. miniglobularis zones) was also characterized by relatively high abundances of agglutinated benthic foraminifera and weakly calcified taxa, the latter belonging to the genera Patellina and Spirillina, at Sites 511 and 1049 (Figs. 4−8). These taxa were clustered in groups 1 and 2 at Sites 511 and 1049, respectively. Even though several of the identified agglutinated species incorporated carbonate particles, detailed SEM-imaging reveals that some of them include dissolution-resistant coccoliths (in comparison with modern foraminiferal calcite; e.g., Honjo & Erez, 1978; Broecker & Clark, 2009; Supplementary Fig. S2). Particularly, within the CIE interval across the Aptian-Albian boundary at Site 511, relative abundances of agglutinated foraminifera increased ∼20% (Fig. 4). Comparable increased abundances of agglutinated foraminifera occurred at the deeper (middle to lower bathyal - 1000−1500 m paleo-water depth) Poggio le Guaine Section within the interval assigned to the Kilian subevent (Coccioni et al., 2014). These patterns suggest that bottom waters in the Atlantic Ocean had relatively low carbonate ion concentrations, hindering calcification of sturdy calcareous benthic foraminiferal species. In fact, ocean acidification during the latest Aptian, likely linked with volcanic CO₂ emissions, was proposed as a trigger for the extinction of heavily calcified late Aptian planktic foraminifera (Matsumoto et al., 2020).

Early Albian epifaunal taxa at Sites 511 (group 2) and 1049 (groups 1 and 3) presented heavily calcified tests, suggesting that carbonate ion saturation increased after the Aptian−Albian boundary. This interpretation is also supported by decreased abundances of agglutinated taxa at both sites during the early Albian, as well as by the highest carbonate contents (averaging 35%) recorded in Site 511 sediments (Fig. 4; Dummann et al., 2021). A second interval of increased abundances of taxa assigned to group 1 (mainly weakly calcified and agglutinated species), coupled with decreased carbonate content, occurred within the upper Ticinella raynaudi/NC10 zones (437.61 to 430.74 mbsf) at Site 511 (Figs. 4, 6). These patterns may suggest a return of reduced carbonate ion saturation conditions at the location of Site 511, although relative abundances of agglutinated taxa did not increase within the upper Ticinella raynaudi/NC10 zones (Fig. 4). Nevertheless, our hypothesized changes in oceanic carbonate chemistry across the AABI should be tested by future high-resolution studies based on geochemical proxies for ocean pH and carbonate ion saturation.

In contrast to trends of planktic foraminiferal assemblages over the AABI, our new high-resolution records at Sites 511 and 1049 show that there was no benthic foraminiferal mass extinction at the Aptian−Albian boundary in the Atlantic Ocean. The benthic foraminiferal records at Sites 511 and 1049 indicate increased organic carbon flux coupled with poorly ventilated and, possibly, carbonate ion-depleted bottom waters in the Atlantic Ocean within the upper Aptian P. rohri and M. miniglobularis zones. Evidence for these changes include increased abundance of infaunal species, and increased abundance of agglutinated and weakly calcified species. However, we did not identify signs of carbonate dissolution during the latest Aptian at Sites 511 and 1049, probably as a consequence of their relatively shallow paleodepths. This latest Aptian paleoceanographic configuration was likely related with massive inputs of volcanic CO₂ into the oceanic and atmospheric reservoirs, leading to ocean acidification and sluggish circulation patterns under globally warming conditions. Deep Atlantic Ocean ventilation and carbonate ion saturation improved during the earliest Albian (lower M. rischi Zone), as evidenced by increased abundances of heavily calcified epifaunal species, followed by high-amplitude oscillations within the mid M. rischi Zone, mostly at Site 1049.

We thank the International Ocean Discovery Program (IODP) for providing the studied samples, and J. Villegas Martín (itt OCEANEON) for helpful discussions regarding cluster analyses. K.G.D.K thanks the Coordination for the Improvement of Higher Education Personnel for financial support (grant IODP/Capes 88887.091703/2014-1 to Gerson Fauth UNISINOS University). Constructive comments by two anonymous referees and the associate editor helped us to significantly improve the first version of the manuscript.

List of benthic foraminiferal species identified at Sites 511 and 1049. We also provide microhabitat assignment for each taxon.

Astacolus calliopsis (Reuss, 1863); epifaunal-infaunal

Astacolus parallelus (Reuss, 1863); epifaunal-infaunal

Astacolus sp. 1; epifaunal-infaunal

Astacolus sp. 2; epifaunal-infaunal

Berthelina flandrini (Moullade, 1960); epifaunal

Berthelina intermedia (Berthelin, 1880); epifaunal

?Citharina spp.; epifaunal-infaunal

Conorotalites sp. 1; epifaunal

Elipsoidella cuneata (Loeblich and Tappan, 1946); infaunal

Fissurina spp.; epifaunal-infaunal

Frondicularia spp.; epifaunal-infaunal

Fursenkoina viscida (Khan, 1950); infaunal

Gavelinella sp. 1; epifaunal

Globulina bucculenta (Berthelin, 1880); epifaunal-infaunal

Globulina lacrima (Reuss, 1845); epifaunal-infaunal

Gyroidinoides infracretaceus Moullade, 1960; epifaunal

Hemirobulina bullata (Reuss, 1860); epifaunal-infaunal

Laevidentalina communis (d'Orbigny, 1826); epifaunal-infaunal

Laevidentalina debilis (Berthelin, 1880); epifaunal-infaunal

Laevidentalina distincta (Reuss, 1863); epifaunal-infaunal

Laevidentalina nana (Reuss, 1863); epifaunal-infaunal

Laevidentalina oligostegia (Reuss, 1845); epifaunal-infaunal

Laevidentalina soluta (Reuss, 1851); epifaunal-infaunal

?Laevidentalina sp. 1 epifaunal-infaunal

Lenticulina muensteri (Roemer, 1839); epifaunal-infaunal

Lenticulina pulchella (Reuss, 1863); epifaunal-infaunal

Lenticulina sp. cf. L. macrodisca (Reuss, 1863); epifaunal-infaunal

Lenticulina sp. 1; epifaunal-infaunal

Lingulina sp. 1; epifaunal-infaunal

Lingulina sp. 2; epifaunal-infaunal

Lingulogavelinella sp. 1; epifaunal

Lingulonodosaria nodosaria (Reuss, 1863); epifaunal-infaunal

Lingulonodosaria sp. 1; epifaunal-infaunal

Marginulinopsis bettenstaedti Bartenstein and Brand, 1951; epifaunal-infaunal

Marginulinopsis sp. aff. M. gracilissima (Reuss, 1863); epifaunal-infaunal

Oolina sp. cf. O. sulcata (Walker and Jacob, 1798); epifaunal-infaunal

Oolina spp.; epifaunal-infaunal

Osangularia schloenbachi (Reuss, 1863); epifaunal

Palaeopolymorphina sp. 1; epifaunal-infaunal

Palmula malakialinensis (Espitalié and Sigal, 1963); epifaunal-infaunal

Patellina subcretacea Cushman and Alexander, 1930; epifaunal

Planularia complanata (Reuss, 1845); epifaunal-infaunal

Pleurostomella reussi Berthelin, 1880; infaunal

Pleurostomella subnodosa Reuss, 1860; infaunal

Pleurostomella sp. 1; infaunal

Praebulimina nannina (Tappan, 1940); infaunal

Praebulimina sp. 1; infaunal

Pseudonodosaria humilis (Roemer, 1841); epifaunal-infaunal

Pseudopatelinella sp. 1; epifaunal

Psilocitharella arguta (Reuss, 1860); epifaunal-infaunal

Psilocitharella recta (Reuss, 1863) epifaunal-infaunal

Pyramidulina zippei (Reuss, 1845); epifaunal-infaunal

Pyramidulina sp. 1; epifaunal-infaunal

Pyramidulina spp.; epifaunal-infaunal

Pyrulina spp.; epifaunal-infaunal

Ramulina tappanae Bartenstein and Brand, 1951; epifaunal-infaunal

Ramulina tetrahedralis Ludbrook, 1966; epifaunal-infaunal

Ramulina sp. 1; epifaunal-infaunal

?Reinholdella hofkeri (Bartenstein and Brand, 1951); epifaunal

Saracenaria triangularis (d'Orbigny, 1840); epifaunal-infaunal

Saracenaria warella Ludbrook, 1966; epifaunal-infaunal

Saracenaria sp. aff. S. pravoslavlevi Fursenko and Polenova, 1950

Saracenaria sp. 1; epifaunal-infaunal

Scheibnerova protindica Quilty, 1984; epifaunal

Spirillina elongata Bielecka and Pozaryska, 1954; epifaunal

Spirillina minima Schacko, 1897; epifaunal

Tristix excavata (Reuss, 1863); epifaunal-infaunal

Vaginulinopsis sp. 1; epifaunal-infaunal

Valvulineria gracillima Ten Dam, 1947; epifaunal

?Valvulineria sp. 1; epifaunal

?Ammobaculites sp. 1; infaunal

Ammodiscus cretaceus (Reuss, 1845); epifaunal-infaunal

Ammodiscus infimus Franke, 1936; epifaunal-infaunal

Ammoglobigerina sp. 1; epifaunal-infaunal

Bathysiphon brosgei Tappan, 1957; epifaunal

Bulbubaculites humei (Nauss, 1947); infaunal

Caudammina crassa (Geroch, 1966); epifaunal

?Cribrostomoides sp. 1; epifaunal-infaunal

Gaudryina dividens Grabert, 1959; infaunal

Glomospira charoides (Jones and Parker, 1860); epifaunal-infaunal

Glomospira gordialis (Jones and Parker, 1860); epifaunal-infaunal

Glomospira irregularis (Grzybowski, 1896); epifaunal-infaunal

Haplophragmoides minor Nauss, 1947; epifaunal-infaunal

Haplophragmoides sp. 1; epifaunal-infaunal

Haplophragmoides sp. 2; epifaunal-infaunal

Kadriayina gradata (Berthelin, 1880); infaunal

Nothia robusta (Grzybowski, 1898); epifaunal

Praedorothia ouachensis (Sigal, 1952); infaunal

Protomarssonella sp. 1; infaunal

Psamosphaera fusca Schulze, 1875; epifaunal-infaunal

Recurvoides spp.; epifaunal-infaunal

Reophax spp.; infaunal

Rhabdammina cylindrica Glaessner, 1937; epifaunal

Rhabdammina sp. 1; epifaunal

?Spiroplectammina sp. 1; epifaunal-infaunal

Spiroplectinella spp.; epifaunal-infaunal

Textulariopsis sp. 1; infaunal

Tritaxia sp. 1; infaunal

Trochammina abrupta Geroch, 1959; epifaunal