Climate variability and relationship with ocean fertility during the Aptian Stage

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a detailed picture of climatic and ocean fertility changes during the Aptian Stage, which are discussed in relation to the direct/indirect role of volcanism.Warm temperatures characterized the pre-OAE 1a interval followed by a maximum warming (of ∼ 2-3 • C) during the early phase of anoxia under intense volcanic activity of the Ontong Java Plateau (OJP).A short-lived (∼ 35 ky) cooling episode interrupted the major warming, following a rapid increase of weathering rates.Nannofossils indicate that eutrophic conditions were reached when temperatures were at their highest and OJP volcanism most intense, thus suggesting that continental runoff, together with increased input of hydrothermal metals, increased nutrient supply to the oceans.The latter part of OAE 1a was characterized by cooling events, probably promoted by CO 2 sequestration during burial of organic matter.In this phase, high productivity was probably maintained by N 2 -fixing cyanobacteria while nannofossil taxa indicating high fertility were rare.The end of anoxia coincided with the cessation of volcanism and a pronounced cooling.The mid-Aptian was characterized by high surface-water fertility and progressively decreasing temperatures, probably resulting from intense continental weathering drawing down pCO 2 .The lowest temperatures, combined with low fertility, were reached in the middle-late Aptian across the interval characterized by blooming of Nannoconus truittii.The data presented suggest that OJP activity played a direct role in inducing global
Although the amount of information about temperature variations across the Aptian is considerable, a complete picture of climatic changes is not available.In most cases, the records are poorly correlated between the different basins and/or cover limited time intervals within the ∼ 12 My-long Aptian Stage (Malinverno et al., 2012).In this work, we focus primarily on surface-water temperatures through the Aptian reconstructed on the basis of calcareous nannofossils from three well sites: Cismon (Italian Southern Alps), Piobbico (Umbria-Marche Basin, central Italy) and DSDP Site 463 (Mid-Pacific Introduction

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Full Mountains).The existing stratigraphic framework for the three sites and available cyclochronology for the Cismon core (Malinverno et al., 2010), allow high-resolution dating of climatic fluctuations.Calcareous nannoplankton live in the (upper) photic zone and are a good proxy of present and past surface-water conditions, being sensitive to temperature, fertility, salinity and pCO 2 (Mutterlose et al., 2005).Extant calcareous nannoplankton occur from coastal areas to the open ocean, although with different abundance and diversity and, together with diatoms, dinoflagellates and bacteria constitute marine phytoplanktonic communities.The Mesozoic geological record confirms the wide geographical/latitudinal distribution of calcareous nannofossils (coccoliths and nannoliths) that are commonly used to trace palaeoecological conditions.Within nannofossil assemblages, nannoconids are inferred to have been restricted to the deep photic zone at the base of the mixed layer on top of the thermocline coinciding with a deep nutricline (Erba, 1994).In the studied intervals, nannoconids are relatively scarce, and micrite mostly consists of coccoliths, thus essentially recording the uppermost water masses.
In this work, stable carbon and oxygen isotopes on bulk rock have been measured to reconstruct changes in surface-water temperature, taking into account potential diagenetic modification.The preservation of nannofossils provides information on the early diagenetic history of pelagic carbonates, (Erba, 1992b;Herrle et al., 2003;Tiraboschi et al., 2009).Although oxygen-isotope ratios contain a mixture of a primary signal and later diagenetic phases (Marshall, 1992), hampering the use of palaeotemperature values, the δ 18 O bulk data can be used to derive trends toward warmer/cooler conditions.New oxygen-isotope data for DSDP Site 463 and Piobbico have been generated, and are directly correlated with calcareous nannofossil variations as well as with new TEX 86 data from the Cismon core.(d) trace the direct/indirect role of volcanism, weathering rates and pCO 2 on climate changes connected with OAE 1a and its aftermath.We also characterize the evolution of surface-water fertility during the Aptian Stage.Previous studies (e.g.Coccioni et al., 1992;Bralower et al., 1993;Erba, 1994Erba, , 2004;;Premoli Silva et al., 1999;Leckie et al., 2002;Mutterlose et al., 2005;Tremolada et al., 2006;Bottini and Mutterlose, 2012) mainly focused on the OAE 1a interval, documenting an increase in surface-water fertility accompanied by high primary productivity, but a record throughout the entire Aptian is missing.We therefore highlight fluctuations in fertility during and after OAE 1a, identifying potential relationships with climatic changes on both the short and the long term as well as oceanic nutrification.

Studied sites
We have investigated the Upper Barremian-Aptian interval at three sites in the Tethys and Pacific Oceans (Fig. 1): The Cismon core, drilled in the Southern Alps, north-eastern Italy (46 • 02 N;

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• 45 E; 398 m altitude) is represented by a total stratigraphic thickness of 131.8 m with 100 % recovery.The site was located on the southern margin of the Mesozoic Tethys, on the eastward deepening slope between the Trento Plateau (a pelagic submarine high) and the Belluno Basin (Erba and Tremolada, 2004).The Cismon sequence was deposited at an estimated palaeo-depth of 1000-1500 m during the Early Cretaceous (Weissert and Lini, 1991;Erba and Larson, 1998;Bernoulli and Jenkyns, 2009).In the uppermost part of the cored section (at 7.80 m) there is a major hiatus corresponding to the late Aptian and the early-middle Albian.The Selli Level (sedimentary expression of OAE 1a) is represented by a ∼ 5 m-thick interval, between 23.67 and 18.64 stratigraphic metre depths (Erba and Larson, 1998;Erba et al., 1999).Lithologically, the Selli Level is characterized by marlstones alternating with black shales and discrete Introduction

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Calcareous nannofossils
Calcareous nannofossils were investigated under polarizing light microscope at 1250X magnification in smear slides and thin-sections.Smear slides were prepared using standard techniques, without centrifuging or cleaning in order to retain the original sedimentary composition.A small quantity of rock was powdered in a mortar with bi-distillate water and mounted on a glass slide with Norland Optical Adhesive.A total of 285 smear slides for the Cismon core, 179 smear slides for the Piobbico core and 281 smear slides for DSDP Site 463 were investigated.At least 300 nannofossil Introduction

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Full specimens were counted in each sample and percentages of single taxa were calculated relative to the total nannoflora.Thin-sections were polished to an average thickness of 7 µm for optimal view of nannofossils; a total of 85 thin-sections for the Cismon Core (which integrates the data from Erba and Tremolada, 2004), 242 for DSDP Site 463, and 179 for Piobbico core were investigated.Absolute abundances were obtained by counting all nannofossil specimens in 1 mm 2 of the thin-section.

Oxygen-isotope analysis
New oxygen stable-isotope analyses were performed at Oxford University on bulk carbonate fraction of 57 samples from DSDP Site 463 and of 373 samples from Piobbico.Bulk-rock samples for isotopic analysis were first powdered, cleaned with 10 % H 2 O 2 followed by acetone, and then dried at 60
Samples were analyzed on an Agilent 1100 series LC/MSD SL.A Prevail Cyano column (150 mm × 2.1 mm, 3 mm) was used with hexane:propanol (99 : 1; vol : vol) as an eluent.After the first 5 min, the eluent increased by a linear gradient up to 1.8 % isopropanol (vol) over the next 45 min at a flow rate of 0.2 mL min −1 .Identification and quantification of the GDGTs isomers was achieved by integrating the peak areas of relevant peaks in m/z 1300, 1298, 1296, 1292, 1050, 1036 and 1022 selected ion monitoring scans.The TEX 86 ratio was calculated following Schouten et al. (2002): where numbers correspond to isoprenoid GDGTs from marine Thaumarchaeota with 1, 2 or 3 cyclopentane moieties, and the crenarchaeol regioisomer has the antiparallel configuration of crenarchaeol (Sinninghe Damsté et al., 2002).Introduction

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Full The TEX 86 values were converted to SST using the most recent core-top calibration as proposed by Kim et al. (2010) for oceans with SST > 15 • C: SST = 38.6 + 68.4 × log (TEX 86 ) . (3) The Branched and Isoprenoid Tetraether (BIT) index is based on the relative abundance of non-isoprenoidal GDGTs derived from soil bacteria versus a structurally related isoprenoid GDGT, "crenarchaeol" with four cyclopentane moieties and one cyclohexane moiety, produced by marine Thaumarchaeota.The BIT index, which thus represents a measure for soil versus marine organic matter input in marine sediments, was calculated according to Hopmans et al. (2004): 4)

Stratigraphic framework
In this work, the stratigraphic framework for the three cores investigated is based on carbon-isotope stratigraphy calibrated with calcareous nannofossil and foraminiferal biostratigraphy.For the Cismon core and DSDP Site 463, magnetic chron CM0 has been used to define the base of the Aptian; this level was not reached with the Piobbico core.
In addition to the two well-known, high-amplitude δ The studied sections cover the latest Barremian to earliest Albian time interval.We revised the Herrle et al. (2004) carbon-isotope segments by extending their codes down to Ap1 and renaming the earliest Albian fluctuations Al1 to Al3.Segments Ap1-Ap7 coincide with previously identified segments C1-C7 (Menegatti et al., 1998).The δ 13 C curve for the rest of the Aptian shows several fluctuations, allowing a higher resolution subdivision into segments Ap8-Ap18.In this paper, we applied a double coding for Ap1/C1 through Ap7/C7 segments and used the new Ap8-Ap18 and Al1-Al3 codes for the late Aptian-earliest Albian time interval.At Cismon, we identify segments Ap1-Ap8, at Piobbico Ap2-Al3, and at DSDP Site 463 Ap1-Ap15.Segments Ap8-Ap15 are less well defined at DSDP Site 463 due to incomplete core recovery.
As far as lithostratigraphy is concerned, the Selli Level or its equivalents are identified at all three sites.A lithological revision of the Piobbico core also allowed the identification of the Kilian Level Equivalent, corresponding to the prominent black shale at the bottom of lithological Unit 12 within core 44 (Erba, 1988).The Kilian level Equivalent in the Piobbico core is characterized by very fine laminations, without bioturbation, and has a thickness of 33 cm (from 45.13 to 44.80 m).Following Petrizzo et al. (2012), the Kilian Level marks the Aptian/Albian boundary.
Regarding the Piobbico core, we identify the presence of a hiatus that eliminates part of the basal Selli Level.In particular, on the basis of the correlation between the lithology and carbon-isotope record from the Piobbico core with the equivalent records from the Cismon core and DSDP Site 463 (Fig. 2), as well as from other sedimentary basins, we note that: (1) the δ 13 C values from 75.94 to 74.80 m, ranging between Introduction

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Full curve.Biscutum constans ranges from 0 to 4.1 % (mean: 0.1 %), D. rotatorius from 0 to 20 % (mean: 2.7 %), and Z. erectus from 0 to 6.5 % (mean: 0.9 %).The latter three species are more abundant corresponding with segments Ap9-Ap11 of the carbonisotope curve.Nannoconids are absent to rare throughout most of the studied interval except for an interval between 60.37 and 55.61 m, where they show rather high abundances (up to 40 % relative abundance in smear slides; 4 × 10 3 specimens mm −2 absolute abundance in thin-section).This particular interval is dominated by N. truittii and coincides with the "N.truittii acme".
At DSDP Site 463 (Fig. 5), the intervals from 624.24 to 623.96 mbsf and from 623.16 to 622.57mbsf, within the Selli Level, are barren of calcareous nannofossils.
For each studied site, two significant factors were extracted from the FA (R-mode) varimax rotation analysis: In the Cismon core (Fig. 6a, Table S1 in the Supplement), Factor 1 (F1) and Factor 2 (F2) represent 32 % of the total variance.F1 (19 % of the total variance) shows the highest positive loadings for W. barnesiae and the highest negative loadings for Introduction

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Full  At DSDP Site 463 (Fig. 6c, Table S3 in the Supplement), F1 and F2 represent 36 % of the total variance.F1 (17 % of the total variance) shows the highest positive loadings for D. rotatorius, Z. diplogrammus, B. constans, R. irregularis and the highest negative loadings for Nannoconus sp.F2 (19 % of the total variance) shows the highest positive loadings for Nannoconus sp., E. floralis, S. stradneri and the highest negative loadings for W. barnesiae, R. asper, B. constans.F1 is interpreted to correspond with surfacewater fertility and F2 with surface-water temperature, respectively.
Nannoconids show apparently a different affinity at Piobbico compared to the Cismon and DSDP Site 463 records, being associated with taxa indicator of warm waters and high nutrients, instead of the cold-water species S. stradneri and E. floralis.However, this discrepancy can be explained with the record of Piobbico starting around the "nannoconid crisis", thus excluding the latest Barremian-earliest Aptian interval dominated by nannoconids.It is well possible that the results of the FA are in this case not reliable or should be considered with caution.
The results of the PCCA analysis are summarized as follow: In the Cismon core (Fig. 6d, Table S4 in

Nannofossil temperature and nutrient indices
On the basis of reconstructed nannofossil affinities to temperature and nutrient content of surface waters, some authors (e.g.Herrle et al., 2003;Tiraboschi et al., 2009) have proposed two indices: the Temperature Index (TI) and the Nutrient Index (NI).According to these palaeoecological reconstructions, and the results of our FA and PCCA analysis, we modified the formulae of Herrle et al. (2003) by excluding taxa that are sparse and rare in the studied sections.The formulae of the indices used here are Eqs.( 4) and ( 5): Ap5/C5.The interval represented by the uppermost part of the Selli Level (segment Ap6/C6), suggests that a pronounced cooling episode was followed by another cold snap after deposition of sediments just above the Selli Level.In the overlying interval (Ap7/C7), the TI shows relatively high-amplitude fluctuations.The NI indicates that the highest surface-water fertility was recorded in the lower part of the Selli Level (segments Ap3/C3 to base of Ap5/C5).The rest of the Selli Level shows low NI.Fertility started to increase in the Ap7/C7 interval.At Piobbico (Fig. 4), the TI shows the warmest temperatures in the lowermost part of the recovered Selli Level corresponding to the base of Ap5/C5.All samples in the Ap5/C5-Ap6/C6 interval are barren of calcareous nannofossils and therefore the TI cannot be used for relative palaeotemperature fluctuations.Corresponding to segments Ap7/C7-Ap9, a general cooling is detected (Ap8), interrupted by a brief warming.Then, from Ap9, a warming continued through most of Ap11.The rest of the late Aptian was characterized by a prolonged cooling episode (from top of Ap11 to top of Ap15) followed, at the end of the Aptian, by a warming trend showing two temperature peaks coinciding with the 113 Level and the Kilian Level at the Aptian/Albian boundary.The earliest Albian (Al1-Al3) shows a brief relative cooling immediately after the Kilian temperature spike, followed by a general warming.
The NI exhibits relatively high values in the interval immediately preceding the Selli Level and in its lowermost portions, corresponding to the base of Ap5/C5.All samples in the Ap5/C5-Ap6/C6 interval are barren of calcareous nannofossils and therefore the NI cannot be used for illustrating palaeofertility fluctuations.Above the Selli Level, a long interval of increased fertility (Ap7-Ap11) shows maximum values in the Ap9-Ap10 interval.The Nannoconus truittii acme is characterized by low surface-water fertility, followed by a relative increase of the NI up to Ap15.The Aptian/Albian boundary interval is marked by a decrease of the NI interrupted by a relative increase through the Kilian Level.The lowermost Albian (Al2-Al3) exhibits a trend to increased fertility extending through the Albian (Tiraboschi et al., 2009).Introduction

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Full At DSDP Site 463 (Fig. 5), the TI indicates warm temperatures just before and at the onset of OAE 1a.The warmest temperatures are reached at the level of segment Ap3/C3.Relative cooling interludes are registered within the Selli Level Equivalent, in segments Ap4/C4 and Ap5/C5.During the late Aptian, a long cooling (Ap7-Ap14) is registered with the coolest temperatures recorded from the top of Ap12 to the base of Ap13.The NI indicates relatively high values in the interval preceding the Selli Level Equivalent.Two maxima are recorded in the Ap3/C3 and at the base of Ap5/C5, respectively.Low NI is detected in the rest of the Selli Level Equivalent.An increase of the NI starts in Ap7 and continues up to the base of Ap12, with a maximum corresponding to Ap8.A decrease in then recorded during the Nannoconus truittii acme interval, followed by a relative increase.

Oxygen-isotope fluctuations
The three oxygen-isotope records are somewhat scattered, and probably reflect a contribution from diagenetic cement.However, δ 18 O trends are reproduced at the three studied sites independently of lithology, and nannofossil preservation is persistently moderate; we conclude, therefore, that the oxygen-isotope records contain a primary palaeotemperature signal only marginally modified by lithification.The main trends (Figs.

TEX 86
A total of 32 samples from the Cismon core have been analyzed for TEX 86 of which 17 contained detectable amounts of GDGTs (Table S7 in the Supplement).TEX 86 data for a number of sediments were excluded as they contained relatively mature organic matter, i.e. the hopane 22S/(22S + 22R) ratio was > 0.2 (van Breugel et al., 2007) at which level TEX 86 values will become biased towards lower temperatures (Schouten et al., 2004).Nearly all sediments have BIT values < 0.3, suggesting relatively low input of soil-derived GDGTs, and thus no bias of the TEX 86 (Weijers et al., 2006).The values obtained for the OAE 1a interval (Figs.changes and detected short-term variability improving the characterization of climate changes during the Aptian.

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Climate variability during OAE 1a
The integration of nannofossil TI and oxygen-isotope data allows the identification of a sequence of synchronous temperature variations labelled A to M (Fig. 8) at the three studied sites.After a warming pulse at the onset of OAE 1a (Interval A), a brief (∼ 35 ky) cooling interlude interrupted warm conditions corresponding to the interval of minimum A cool snap across segment Ap4/C4 interrupting the main warming has also been detected in the Vocontian Basin (Kuhnt et al., 2011;Lorenzen et al., 2013), Tethys (Menegatti et al., 1998;Luciani et al., 2001;Stein et al., 2011), and Turkey (Hu et al., 2012).
The correlation of oxygen-isotope and calcareous nannofossil datasets with SST estimates from TEX 86 is difficult since the TEX 86 data available for OAE 1a have a much lower resolution and provide relatively scattered records.The new TEX 86 data for the Cismon core are suggestive of SSTs ranging between 22 and 27 • C. The lowermost  We notice that although the temperature variability (∆ = ∼ 4-5 • C) is similar in all sites, at Cismon the absolute temperatures are generally 5 to 8 • C lower than at DSDP Site 463, Shatsky Rise and Lower Saxony Basin.For Cismon, also the highest (coolest temperature) δ 18 O values are ∼ 1 ‰ greater than those registered at DSDP Site 463 and ∼ 0.5 ‰ greater than those at Piobbico.Generally cooler temperatures for Cismon could be explained by different latitudinal settings, the Cismon site being at ∼ 30 • N, the Shatsky Rise at an almost equatorial position and the DSDP Site 463 at ∼ 20 • S.However, this seems not to apply to the Boreal section (39 • N) characterized by the highest (∼ 35 • C) SST.Another possible explanation for this discrepancy may be that the TEX 86 values from the Cismon core are already affected by the higher level of thermal maturity (i.e.hopane 22S/(22S + 22R) ratios of 0.1-0.2).It has been documented that destruction of GDGTs during thermal maturation processes results in lower TEX 86 values due to the fact that GDGTs with cyclopentane moieties are thermally less stable (Schouten et al., 2004).Finally, it has been shown in several modern settings that Introduction

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Full TEX 86 , although calibrated against sea-surface temperature, may sometimes reflect changes in subsurface water temperatures as well (e.g.Huguet et al., 2007;Lopes dos Santos et al., 2010), possibly because the source organisms, Thaumarchaeota, also reside in the deeper thermocline where nutrients such as ammonia might be available.

Long-and short-term changes in surface-water fertility
The nannofossil NI exhibits similarities between the three studied sites (simplified in Fig. 7, where nannofossil data are calibrated against the δ 13 C curve, adopting the timescale of Malinverno et al., 2012).The earliest Aptian (segments Ap1/C1 and Ap2/C2) is characterized by low to intermediate NI values suggestive of oligotrophic conditions.The onset of OAE 1a was marked by increasing fertility, which reached a maximum in the core interval of the negative carbon-isotope excursion (segment Ap3/C3).A decrease in surface-water fertility characterized the rest of the Selli Level (segments Ap4/C4-Ap6/C6).A shift to meso-to eutrophic conditions is detected from segment Ap8 to the beginning of the N. truittii acme interval, corresponding to minimal fertility conditions.The latest Aptian is then characterized by intermediate NI values, continuing into the earliest Albian.
The early Aptian has been generally seen as a time of warm and humid climate, mainly responsible for accelerated continental weathering, and consequent important nutrient fluxes to the ocean sustaining high productivity (e.g.Leckie et al., 2002;Erba, 2004;Föllmi, 2012).It has also been proposed that higher fertility in the global ocean was triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes (e.g.Larson and Erba, 1999;Leckie et al., 2002;Erba, 2004).
Peaks in the NI are detected at the levels of the "nannoconid decline" (∼ 1 Ma before OAE 1a) and the "nannoconid crisis".This relationship is in agreement with the interpretation of nannoconids as oligotrophic taxa, which suffered during episodes of increased surface-water fertility.The results of the FA and PCCA analysis also support this affinity for nannoconids.Their virtual absence during the early phase of OAE 1a has been Introduction

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As far as the OAE 1a interval is concerned, the fluctuations in surface-water fertility reconstructed in our work are in agreement with other studies on calcareous nannofossils from the Tethys, Boreal Realm and Atlantic Ocean.Furthermore, other proxies, for example palynomorphs (Hochuli et al., 1999) and phosphorus (e.g.Föllmi et al., 2006;Föllmi and Gainon, 2008;Stein et al., 2012), support this interpretation.

Climate and environmental changes and their relation to igneous-tectonic events during the Aptian
Our data confirm a relationship between major volcanic episodes and climate change, with associated (or subsequent) perturbations in ocean chemistry, structure and fertility.Specifically, the construction of the OJP LIP, documented in the Os-isotopic record (Tejada et al., 2009;Bottini et al., 2012), Pb isotopes (Kuroda et al., 2011), and biomarkers (Méhay et al., 2009), suggestive of a stepwise accumulation of volcanogenic CO 2 in the atmosphere (Fig. 8), correlates in time with OAE 1a and was marked by global warming at the onset of the mid-Cretaceous greenhouse (e.g.Larson and Erba, 1999;Jenkyns, 2003).A short-lived event of possible methane hydrate dissociation probably promoted a ∼ 100 kyr-long interval of accelerated continental weathering, temporarily reducing the CO 2 concentrations and inducing a subsequent cooling interlude (∼ 35 ky).The next interval, marked by a maximum warming, coincided with the beginning of the most intense volcanic phase of OJP (Bottini et al., 2012).This correspondence is suggestive of a (super)greenhouse climate triggered by excess volcanogenic CO 2 .The rest of OAE 1a was accompanied by climate variability including cooling interludes.Termination of widespread anoxia-dysoxia coincided with the end of the main emplacement of the OJP (Bottini et al., 2012) and a major cooling.

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Full Large-scale igneous-tectonic events took place also during the late Aptian, but their causal impact on climate changes are less obvious, since palaeotemperatures were generally cooler.We notice that submarine volcanism (construction of OJP, Manihiki Plateau, Hikurangi Plateau, the early phase of Kerguelen Plateau) correlates with global warming, but subaerial volcanism (Kerguelen Plateau LIP) was associated with relatively cool conditions (Fig. 7).In addition to magmatic fluxes of different orders of magnitude (lower for Kerguelen, see Eldholm and Coffin, 2000), subaerial volcanism probably injected ashes and gases into the atmosphere inducing short-term cooling associated with individual degassing phases.Feedbacks related to atmospheric CO 2 drawdown via accelerated weathering were probably most significant, as also suggested by Ca isotopes (Blättler et al., 2012).On the basis of pedogenic calcretes from South Korea, Hong and Lee (2012) documented a decrease in CO 2 concentrations from ∼ 1000 to ∼ 500 ppmV for an interval in the late Aptian corresponding, as discussed above, to relatively cooler temperatures.However, these data present a large uncertainty in the age assignment.Recently published data by Li et al. (2014), from similar continental facies in south-east China spanning the same age, are suggestive of higher values.Specifically, here we present (Fig. 7) a revised plot of Li et al. (2014) data based on the Malinverno et al. (2012)  Volcanically linked climate change seems closely connected to nutrient recycling and ocean fertilization.Different eruption styles and duration, as well as magma composition and quantity, presumably produced diverse weathering rates and introduction of biolimiting metals.Although calcareous nannoplankton are but one group of primary producers and they thrive under oligotrophic-mesotrophic conditions, the nannofossil NI can be used to trace the trophic levels of surface waters in the past.Figure 7 suggests that nutrient availability was strongly coupled with climate change in the early Aptian, but less so in the late Aptian.Fertility fluctuations could be due to differential weathering rates.During OAE 1a, greenhouse conditions generated by repetitive volcanogenic CO 2 emissions (e.g.Méhay et al., 2009;Erba et al., 2010) might have increased weathering rates, and thereby the supply of nutrients.We see a correspondence between maximum warming and high surface-water fertility.In addition, the largest submarine volcanic pulses at the beginning of OAE 1a and in the mid-late Aptian seem to have introduced biolimiting metals during submarine plateau construction.The nutrients presumably stimulated primary productivity with consequent consumption of oxygen through organic matter and metal oxidation, hence promoting anoxic conditions.The upper part of the Selli Level has high TOC content, indicating that productivity and/or preservation of organic matter was relatively high.The apparent oligotrophic conditions suggested by the NI are explained by biomarker data and nitrogen stable isotopes, indicating N-fixing cyanobacteria as the likely main primary producers during OAE 1a (Kuypers et al., 2004;Dumitrescu and Brassell, 2006).Their N-fixation potentially provided N nutrients for the rest of the oceanic biota and presumably was

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Full the key-process in the production of organic matter, maintaining higher productivity through OAE 1a.The accumulation and burial of organic matter would have progressively acted as storage for excess CO 2 , leading to lower temperatures and, possibly, to the termination of OAE 1a under less active (or ceased) OJP volcanism.We notice that the two more intense cooling interludes across OAE 1a correspond to levels with relatively high TOC content (> 4 %), suggesting that the burial of organic matter may have acted as a reservoir for excess CO 2 , thus temporarily mitigating greenhouse conditions.Among Cretaceous calcareous nannofloras, nannoconids are interpreted as specific to the lower photic zone, associated with a deep nutricline, so that they thrived when surface waters were characterized by oligotrophic conditions (Erba, 1994(Erba, , 2004)).The record of nannoconid distribution compared with the nannofossil NI confirms this hypothesis for the entire Aptian interval: the "nannoconid crisis" correlates with an increase of the NI, while the return of nannoconids following deposition of the Selli Level and the N. truittii acme corresponds to minima in the NI curve.We stress the fact that nannoconid abundance does not unequivocally correlate with climate change, at least in the Aptian, because the "nannoconid crisis" coincides with major warming while the final nannoconid disruption (the end of the N. truittii acme) corresponds to the most severe cooling.
These data contradict the interpretation of McAnena et al. (2013) for the nannoconid failure due to cold conditions in the late Aptian and imply a different explanation for abundance changes of these rock-forming nannofossils.We believe that volcanically induced CO 2 concentrations played a key role for nannoconid calcification, regardless of climatic conditions (Erba, 2006).Both the OJP and Kerguelen LIPs emitted huge quantities of CO 2 that arguably provoked ocean acidification.We emphasize that the prolonged cooling in the late Aptian promoted CO 2 absorption in the ocean and acidification.The nannoconid crises, including their final collapse in the latest Aptian, could thus be viewed as failures in biocalcification.Similarly, the major reduction in size, decrease in abundance, and species turnover documented for planktonic foraminifers

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Full  (Huber and Leckie, 2011), which is coeval with the final nannoconid decline and a nannofossil turnover, might be the response of calcareous zooplankton to volcanically triggered ocean acidification.

Conclusions
Quantitative study of calcareous nannofossils integrated with oxygen-isotope and TEX 86 records from the Tethys and Pacific Oceans has provided a reconstruction of the climatic evolution through the entire Aptian.The excellent stratigraphic time control on the studied sections coupled with high sampling density, allows confirmation of some of the climatic variations detected in previous work and highlights, during OAE 1a, temperature fluctuations not previously detected.
The results from the Tethys and Pacific Oceans confirm climatic variability through the Aptian, characterized by a warming trend that began prior to and reached a maximum during OAE 1a, coincident in time with the negative carbon-isotope excursion.The rest of OAE 1a was marked by subsequent cold snaps and a further cooling took place when the uppermost part of the Selli Level was being deposited.A cooling marked the end of global anoxia and another long-lasting cooling characterized the middle late Aptian, culminating soon after the N. truittii acme.The latest Aptian was, instead, characterized by a gradual warming recorded by nannofossil assemblages and TEX 86 data.SSTs from TEX 86 are suggestive of 24-27 • C in the Tethys during OAE 1a, which are nevertheless 5 to 8 • C lower than estimates from the Pacific Ocean and Boreal Realm, being probably affected by maturity levels or other factors.Although the earliest Aptian was characterized by oligotrophic conditions, the onset of OAE 1a was marked by increasing fertility, which reached a maximum at a time corresponding to the core of the negative carbon-isotope excursion.A decrease in surface-water fertility is recorded from the younger part of the Selli Level.A shift to warm and mesoto eutrophic conditions is detected after OAE 1a up to the beginning of the N. truittii Introduction

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Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The aims of our work are to: (a) trace climatic variations during the Aptian Stage; (b) reconstruct, in high resolution, the climate variability through OAE 1a; (c) identify synchroneity and diachroneity of temperature variations in different oceanic basins; Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 13 C Aptian excursions, several minor fluctuations are identified in the carbon-isotope record from the Tethys, Pacific and Atlantic Oceans, which allow codification of major and minor perturbations.Menegatti et al. (1998) focused on the late Barremian-early Aptian interval and identified segments C1-C8.Subsequently, Bralower et al. (1999) extended the codification of Menegatti et al. (1998) through the rest of the Aptian Stage (C1-C11).Herrle et al. (2004) introduced new codes for the Aptian starting from Ap6, coinciding with C5 and C6 of Menegatti et al. (1998), to Al2.McAnena et al. (2013) used the Ap9-Al3 segments previously identified by Herrle et al. (2004).Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |negative excursion Ap3/C3 where values of Cismon and DSDP Site 463 sediments are below 1 ‰; (2) δ 18 O values from 75.94 to 74.80 m fall between −3 and −1 ‰ and never reach the highly negative values (−4 ‰) characteristic of those in the Ap3/C3 segment of the other two sites, but rather conform to the range of values detected in segments Ap4/C4 and Ap5/C5; (3) the Selli Level in the Cismon core is characterized by three lithological sub-units (Erba et al., 1999), the lowermost being represented by laminated black shales corresponding to segment Ap3/C3, the second characterized by prevailing light grey marlstones corresponding to segments Ap4/C4 and Ap5/C5, and the uppermost one characterized by laminated black shales corresponding to segment Ap6/C6.The total organic carbon content (TOC) in the Cismon core shows highest values corresponding to segments Ap4/C, the base of segment Ap5/C5, as well as segment Ap6/C6.Similar high values are detected at DSDP Site 463 in coeval stratigraphic positions.At Piobbico, only two lithological sub-units are recognized

Figures 3 -
Figures 3-5 illustrate the distribution of the high-fertility (D. rotatorius, B. constans, Z. erectus) and low-fertility (W.barnesiae) nannofossil taxa (following Roth and Krum-bach, 1986; Premoli Silva et al., 1989a, b; Watkins, 1989;Coccioni et al., 1992; Erba et  al., 1992b;Williams and Bralower, 1995;Bellanca et al., 1996; Herrle, 2002Herrle, , 2003;; Her- Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | the Supplement), the first component (22 % of the total variance) shows the highest positive loadings for D. rotatorius, B. constans, Z. erectus and the highest negative loadings for W. barnesiae and Nannoconus sp.The Discussion Paper | Discussion Paper | Discussion Paper |second component (15 % of the total variance) shows the highest positive loadings for S. stradneri, W. barnesiae and the δ 18 O (associated variable), and the highest negative loadings for R. asper and C. surirellus.The 1 axis is interpreted to correspond with surface-water fertility, the 2 axis to surface-water temperature.In the Piobbico core (Fig. 6e, Table S5 in the Supplement), the first component (28 % of the total variance) shows the highest positive loadings for D. rotatorius, B. constans, Z. erectus and the highest negative loadings for Nannoconus sp.The second component (11 % of the total variance) shows the highest positive loadings for E. floralis, S. stradneri, W. barnesiae and the highest negative loadings for D. rotatorius.The associated variable δ 18 O has loadings close to zero.The 1 axis is interpreted to correspond with surface-water fertility, while the interpretation for the 2 axis is not straightforward, but might correspond with surface-water temperature.The dataset collected in this work for the Piobbico core has been integrated with the dataset fromTiraboschi et al. (2009) covering the Albian.The results of the PCCA analysis performed on the integrated dataset are presented in Fig.6f.The first component (32 % of the total variance) shows the highest positive loadings for D. rotatorius, B. constans, Z. diplogrammus, R. asper, C. surirellus, R. irregularis, Z. erectus and the highest negative loadings for W.barnesiae and Nannoconus sp..The second component (13% of the total variance) shows the highest positive loadings for E. floralis, S. stradneri and R. parvidentatum and the highest negative loadings for R. asper, Z. diplogrammus and B. constans.Also the associated variable δ 18 O exhibits positive loadings.The 1 axis is interpreted to correspond with surface-water fertility, while the 2 axis corresponds with surface-water temperature.At DSDP Site 463 (Fig.6g, TableS6in the Supplement), the first component (24 % of the total variance) shows the highest negative loadings for R. asper, Z. erectus, and lower negative loadings for S. stradneri, E. floralis.The second component (16 % of the total variance) shows the highest positive loadings for D. rotatorius, R. irregularis, B. constans, Z. erectus, and negative loadings for W. barnesiae, and Nannoconus sp., Discussion Paper | Discussion Paper | Discussion Paper |δ18 O (associated variable) has loadings close to zero.The 1 axis is interpreted to correspond with surface-water temperature, the 2 axis with surface-water fertility.
NI = (Bc + Dr + Ze)/(Bc + Dr + Ze + Wb) × 100 (6) where: Ss = S. stradneri; Ef = E. floralis; Rp = R. parvidentatum; Ra = R. asper; Zd = Z.diplogrammus; Bc = B. constans; Dr = D. rotatorius; Ze = Z.erectus; Wb = W. barnesiae.The nannofossil TI, calibrated against carbon-isotope stratigraphy, has revealed systematic and synchronous changes in the Cismon core, Piobbico core and at DSDP Site 463.A complete nannofossil record through OAE 1a is available only for the Cismon core, because the Selli Level of the Piobbico core is incomplete and many samples are barren of nannofossils, while at DSDP Site 463 the top of the Selli Level Equivalent is probably not recovered and some samples are barren.In the three investigated sites, the TI and NI show the following fluctuations: At Cismon (Fig. 3) the TI shows high-frequency fluctuations superimposed on a longer term trend.The warmest temperatures were reached in the early phase of Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3-5) are summarized as follows: Along segments Ap1/C1 and Ap2/C2 the isotopic ratios are relatively stable, being ∼ −2 ‰ at DSDP Site 463, −1.5 ‰ at Piobbico and −1 ‰ at Cismon.At the end of segment Ap2/C2 values start to decrease, reaching −4 ‰ in correspondence with the negative carbon-isotope excursion (segment Ap3/C3).At Cismon, the decreasing trend is interrupted by a short-lived (∼ 35 ky) interval of higher values (−1.5 ‰).At segment Ap4/C4, the δ 18 O values start increasing and in the middle part of the Selli Level (segment Ap5/C5) they fluctuate: between −1 and −2 ‰ at Cismon, between −1 and −3 ‰ at Piobbico, and between −1 and −4 ‰ at DSDP Site 463.Corresponding with segment Ap6/C6, δ 18 O values are relatively stable between −1 and −2 ‰.Starting from segment Ap7/C7, oxygen isotopes illustrate progressively increasing ratios Discussion Paper | Discussion Paper | Discussion Paper | reaching ∼ −1 ‰ around the Nannoconus truittii acme interval and then decrease to a minimum of ∼ −3 ‰ close to the Aptian/Albian boundary.With respect to coeval sediments in the Tethys and Pacific Ocean, we notice that the oxygen-isotope values of the Cismon are greater by 1 ‰.
2 and 8) comprise one sample having a TEX 86 value of 0.57 indicative of ∼ 22 • C sea-surface temperature (SST) and corresponding to the most negative δ 13 C values (segment Ap3/C3).The rest of segment Ap3/C3 is characterized by values from 0.67 to 0.61 (SST = ∼ 24 to 27 • C).Segments Ap4/C4 and Ap5/C5 are characterized by relatively stable values lying between 0.66 and 0.64 (SST = ∼ 25-26 • C).Corresponding to segment Ap6/C6, one sample gives a TEX 86 value of 0.58 (SST = ∼ 22.5 • C) and the following one of 0.64 (SST = ∼ 25.5 • C). -and short-term temperature fluctuations during the Aptian Long-term temperature variations are comparable with the results of previous studies based on various temperature proxies (i.e.calcareous nannofossils, palynomorph, oxygen isotopes, and TEX 86 ).The high-resolution sampling and the stratigraphic calibration of the studied sections, enabled better constraints on long-term temperature Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

δ
13 C values (Interval B).It was followed by a maximum warming in the core of segment Ap3/C3 (Interval C).A cooling episode (Interval D) coincides with segment Ap4/C4 and base Ap5/C5.Intermediate climatic conditions, including one minor cooling episode (Interval E), a warm interlude (Interval F) and another minor cooling (Interval G), characterize segment Ap5/C5.Warmer temperatures (Interval H) preceded a more prominent cooling (Interval I) correlating with the latest part of OAE 1a and corresponding with segment Ap6/C6.The end of anoxia was marked by a short-lived warming (Interval L) and a further major cooling (Interval M) coinciding with the onset of segment Ap7/C7.
data point, which corresponds to Interval B, indicates an SST of ∼ 22 • C which is the coolest value for the studied interval and well matches with cooler conditions reconstructed from other data.The SST values for the following three data points are rather puzzling: two indicate temperatures of ∼ 23-25 • C and fall in Interval C -the warmest of OAE 1a -while the third data point shows almost 27 • C although it falls in Interval D, interpreted to correspond to a time of relative cooling.The rest of the samples, encompassing Intervals E to H, and representing minor temperature fluctuations, fall between Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | time scale, which indicate progressively increasing CO 2 concentrations from ∼ 1000 up to ∼ 2000 ppmV across the N. truittii acme interval despite a general cooling trend.In correspondence of the lowest temperatures reached in the latest Aptian, CO 2 estimates decrease to ∼ 1600 ppmV.The following early Albian warming trend was instead paralleled by increasing CO 2 concentrations up to 2600 ppm.The long-lasting cool conditions of the late Aptian have been recently quantified using TEX 86 data (McAnena et al., 2013), which indicate a total decrease of palaeotemperatures of ∼ 4 • C (from 32 to 28 • C) in the equatorial Proto-Atlantic Ocean followed by a warming (∼ 4 • C) linked to the earliest Albian OAE 1b.The trends of our nannofossil TI curve are similar to these TEX 86 -reconstructed SST changes (Fig. 7), so we adopt the values of McAnena et al. (2013) to estimate climate variations through the Aptian Discussion Paper | Discussion Paper | Discussion Paper | using the nannofossil TI fluctuations.The warming at the onset of OAE 1a corresponds to an increase of 2-3 • C and climate variability during OAE 1a is marked by a cooling of ∼ 2 • C. The prominent cooling at the end of global anoxia corresponds to a decrease of ∼ 3 • C, followed by a warming of ∼ 3 • C and the coolest interval in the late Aptian is marked by a further decrease of ∼ 4 • C. As far as OAE 1a is concerned, the SST variability estimated from the TI (2-3 • C) differs little from the direct TEX 86 estimates of 4-5 • C.

Figure 4 .
Figure 4. Piobbico core: fluctuations of calcareous nannofossil temperature and fertility indicator taxa.Temperature (TI) and Nutrient (NI) indices based on calcareous nannofossils (low values of the TI indicate high temperatures and vice versa; high values of the NI indicate high surface-water productivity and vice versa).Orange curve indicates smoothed TI and NI records based on three-point moving average.δ 13 C is from Erba et al. (under final review by the editor).Nannofossil and foraminiferal biostratigraphy is from Erba et al. (1988) and Tornaghi et al. (1989).Bulk δ 18 O data are from this work.

Fig. 4 .
Fig. 4. Piobbico core: fluctuations of calcareous nannofossil temperature and fertility indicator taxa.Temperature (TI) and Nutrient (NI) indices based on calcareous nannofossils (low values of the TI indicate high temperatures and vice versa; high values of the NI indicate high surfacewater productivity and vice versa).Orange curve indicates smoothed TI and NI records based on three-point moving average.δ 13 C is from Erba et al. (2014).Nannofossil and foraminiferal biostratigraphy is from Erba et al. (1988) and Tornaghi et al. (1989).Bulk δ 18 O data are from this work.
Tornaghi et al. (1989) km west of the town of Piobbico (Marche, Italy), at km 33 of the Apecchiese State Road No. 257, on the left hydrographic side of the Biscubio stream.Coring penetrated the entire Marne a Fucoidi Formation, including the upper transition to the Scaglia Bianca and the lower transition to the Maiolica.The total length of the core is 84 m with 98.8 % recovery; after adjusting for dip direction, the stratigraphic thickness equals 77 m.The lithostratigraphy and calcareous plankton biostratigraphy of the core were described byErba (1988Erba ( , 1992a) )andTornaghi et al. (1989).The Selli Level, consisting of black shales and radiolarian-rich beds, extends from 75.94 to 73.47 m.The interval studied covers the interval from 77 to 40 m.DSDP Site 463 was drilled at a water depth of 2525 m on the ancient structural high of the western Mid-Pacific Mountains (21 • 35 3.78 N; 12• • 21.01 N, 174 • 40.07 E) during DSDP Leg 62.
• C. Powders were then reacted with purified Introduction D. rotatorius, B. constans, Z. erectus.F2 (13 % of the total variance) shows positive loadings for W. barnesiae, S. stradneri, E. floralis, and negative loadings for R. asper, C. surirellus and Z. diplogrammus.F1 is interpreted to correspond with surface-water fertility and F2 to surface-water temperature, respectively.In the Piobbico core (Fig.6b, TableS2in the Supplement).F1 and F2 represent 36 % of the total variance.F1 (25 % of the total variance) shows high positive loadings for Z. erectus, D. rotatorius, B. constans, R. irregularis, C. surirellus, R. asper and high negative loadings for Nannoconus sp. and W. barnesiae.F2 (11 % of the total variance) shows the highest positive loadings for W. barnesiae, S. stradneri, E. floralis and the highest negative loadings for Nannoconus sp.F1 is interpreted to correspond with surface-water fertility and F2 with surface-water temperature, respectively.
25 and 27 • C. We identify one more discrepancy in the relatively low estimated SST (22.5 • C) for one sample falling in Interval H, suggested by TI and oxygen isotopes to be a relatively warm interlude.Another problem of the TEX 86 data of the Cismon core is related to the SSTs, which are ∼ 5 to 8 • C lower compared with the TEX 86 records from other sites.TEX 86 data from DSDP Site 463 cover the Ap4/C4 to Ap6/C6 interval and range between 31 and 34 • C (recalibrated from Schouten et al., 2003 using Eq. 2).At Shatsky Rise, temperatures fall between 30 and 35 • C (recalibrated values from Dumitrescu et al., 2006 using Eq. 2).Here, two cooling interludes are detected: the first cooling of 4 • C down to 30 • C was during segment Ap4/C4 and corresponds to the cooling of our Interval D. The second cooling of 5 • C down to 30 • C seems to corresponds to segment Ap6/C6 and possibly reflects the cooling of Interval I.In the Lower Saxony Basin, SSTs indicate a distinctive warming during OAE 1a (segment C3-C6), with TEX 86 temperature estimates of 31-34 (Mutterlose et al., 2010)the interval following OAE 1a (the C7 segment) reveal stable SSTs around 30• C(Mutterlose et al., 2010).