Laia Alegret | University of Zaragoza (original) (raw)

Papers by Laia Alegret

Science, Jan 1, 2010

Leg 207 Site 1259C and the Brazos River Cretaceous-Paleogene (K-Pg) sections was determined at th... more Leg 207 Site 1259C and the Brazos River Cretaceous-Paleogene (K-Pg) sections was determined at the University of Erlangen on wet powdered samples (grain size <10 !m obtained with a McCrone Micromill) (S1) with a Siemens D5000 X-ray diffractometer.

An abrupt environmental disruption occurred in the photic zone and at the seafl oor during the mi... more An abrupt environmental disruption
occurred in the photic zone and at the
seafl oor during the mid-Paleocene biotic
event (MPBE). Calcareous nannoplankton,
planktic foraminifer, and benthic foraminifer
assemblages at Zumaia section (western
Pyrenees) underwent a rapid and remarkable
transformation. The major calcareous
plankton assemblage changes suggest a
shift from relatively cooler mesotrophic to
warmer, more oligotrophic conditions, indicating
a disturbed environment due to the
warming of the ocean. Benthic foraminifer
assemblages were also signifi cantly affected
by the MPBE; diversity of the assemblages
and buliminids show net decline and the
low food and opportunistic taxa increase in
abundance. The reorganization of the planktic
ecosystem possibly involved changes
in the food fl ux (type and quantity) to the
seafl oor, thus triggering changes in the benthic
communities.
A 1‰ negative δ13C shift and a 30% carbonate
content decrease are recorded in connection
with the biotic event. This suggests
that during the MPBE, as in the Paleocene-
Eocene Thermal Maximum (PETM), an
input of a large mass of isotopically depleted
carbon into the ocean and atmosphere could
have lowered the deep-sea pH, triggering a
rapid shoaling of the lysocline and contributing
to greenhouse warming.
The MPBE was short lived: according to
the counting of limestone-marl couplets, the
stratigraphic expression of precession cycles
throughout the Zumaia section, the MPBE
lasted for ~52–53 k.y., with the core of the
event representing ~10–11 k.y.
The Zumaia section is the fi rst land-based
locality in which the MPBE is recognized
and described in detail. Due to its expanded
character and excellent paleontological
record, this section may prove to be a global
reference section for the study of this shortlived
event.

The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, ... more The impact of an asteroid at the Cretaceous/Paleogene (K/Pg) boundary triggered dramatic biotic, biogeochemical and sedimentological changes in the oceans that have been intensively studied. Paleo-biogeographical differences in the biotic response to the impact and its environmental consequences, however, have been less well documented. We present a high-resolution analysis of benthic foraminiferal assemblages at Southern Ocean ODP Site 690 (Maud Rise, Weddell Sea, Antarctica).
At this high latitude site, late Maastrichtian environmental variability was high, but benthic foraminiferal assemblages were not less diverse than at lower latitudes, in contrast to those of planktic calcifiers. Also in contrast to planktic calcifiers, benthic foraminifera did not suffer significant extinction at the K/Pg boundary, but show transient assemblage changes and decreased diversity. At Site 690, the extinction rate was even lower (~3%) than at other sites. The benthic foraminiferal accumulation rate varied little across the K/Pg boundary, indicating that food supply to the sea floor was affected to a lesser extent than at lower latitude sites. Compared to Maastrichtian assemblages, Danian assemblages have a lower diversity and greater relative abundance of heavily calcified taxa such as Stensioeina beccariiformis and Paralabamina lunata. This change in benthic foraminiferal assemblages could reflect post-extinction proliferation of different photosynthesizers (thus food for the benthos) than those dominant in the Late Cretaceous, therefore changes in the nature rather than in the amount of the organic matter supplied to the seafloor. However, severe extinction of pelagic calcifiers caused carbonate supersaturation in the oceans, thus might have given competitive advantage to species with large, heavily calcified tests. This indirect effect of the K/Pg impact thus may have influenced the deep-sea dwellers, documenting the complexity of the effects of major environmental disturbance.

The benthic foraminiferal turnover and extinction event (BEE) associated with the negative carbon... more The benthic foraminiferal turnover and extinction event (BEE) associated with the negative carbon isotope excursion (CIE) across the Paleocene–Eocene Thermal Maximum (PETM), is analyzed in the Zumaia section (Spain), one of the most complete and expanded deep-water sequences known worldwide. New biostratigraphic, paleoecologic, and paleoenvironmental data on benthic foraminifera are correlated to information on planktic foraminiferal and calcareous nannofossil turnover in order to evaluate possible causes and consequences of the PETM. Gradual but rapid extinction of 18% of the benthic foraminiferal species starts at the onset of the CIE, after the initial ocean warming (as inferred from calcareous nannofossils) recorded in the last 46 kyr of the Paleocene. This gradual extinction event culminated ~10.5 kyr after the onset of the CIE and led to the main BEE, affecting 37% of the species. Therefore, extinctions across the PETM affected a total of 55% of the benthic foraminiferal species at Zumaia. The gradual extinction occurred under inferred oxic conditions without evidence for carbonate dissolution, indicating that carbonate corrosivity and oxygenation of the ocean bottom waters were not the main cause of the event. An interval characterized by dissolution occurs above the main BEE, indicating that bottom waters became corrosive after the main extinction. Carbonate is progressively better preserved through the overlying deposits, indicating a gradual return of carbon isotope values to background levels, consistent with a slow deepening of the carbonate compensation depth after its initial rise due to abrupt acidification of the oceans. Microfossil data support a rapid onset of the PETM, followed by long-term effects on calcareous plankton and benthic foraminifera.

An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms... more An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms are not well understood. The collapse of sea-surface to sea-floor carbon isotope gradients has been interpreted as reflecting a global collapse of primary productivity (‘Strangelove Ocean’) or export productivity (‘Living Ocean’), which caused mass extinction higher in the marine food chain. Phytoplankton-dependent benthic foraminifera on the deep-sea floor, however, did not suffer significant extinction, suggesting that export productivity persisted at a level sufficient to support their populations. We compare benthic foraminiferal records with benthic and bulk stable carbon isotope records from the Pacific, Southeast Atlantic and Southern Oceans. We conclude that end-Cretaceous decrease in export productivity was moderate, regional, and insufficient to explain marine mass extinction. A transient episode of surface ocean acidification may have been the main cause of extinction of calcifying plankton and ammonites, and recovery of productivity may have been as fast in the oceans as on land.

Marine Micropaleontology, 2007

Marine Micropaleontology, 2009

Deep-sea benthic foraminifera show important but transient assemblage changes at the Cretaceous/ ... more Deep-sea benthic foraminifera show important but transient assemblage changes at the Cretaceous/ Paleogene (K/Pg) boundary, when many biota suffered severe extinction. We quantitatively analyzed benthic foraminiferal assemblages from lower bathyal-upper abyssal (1500-2000 m) northwest Pacific ODP Site 1210 (Shatsky Rise) and compared the results with published data on assemblages at lower bathyal (~1500 m) Pacific DSDP Site 465 (Hess Rise) to gain insight in paleoecological and paleoenvironmental changes at that time. At both sites, diversity and heterogeneity rapidly decreased across the K/Pg boundary, then recovered. Species assemblages at both sites show a similar pattern of turnover from the uppermost Maastrichtian into the lowermost Danian: 1) The relative abundance of buliminids (indicative of a generally high food supply) increases towards the uppermost Cretaceous, and peaks rapidly just above the K/Pg boundary, coeval with a peak in benthic foraminiferal accumulation rate (BFAR), a proxy for food supply. 2) A peak in relative abundance of Stensioeina beccariiformis, a cosmopolitan form generally more common at the middle than at the lower bathyal sites, occurs just above the buliminid peak. 3) The relative abundance of Nuttallides truempyi, a more oligotrophic form, decreases at the boundary, then increases above the peak in Stensioeina beccariiformis. The food supply to the deep sea in the Pacific Ocean thus apparently increased rather than decreased in the earliest Danian. The low benthic diversity during a time of high food supply indicates a stressed environment. This stress might have been caused by reorganization of the planktic ecosystem: primary producer niches vacated by the mass extinction of calcifying nannoplankton may have been rapidly (b10 kyr) filled by other, possibly opportunistic, primary producers, leading to delivery of another type of food, and/or irregular food delivery through a succession of opportunistic blooms. The deep-sea benthic foraminiferal data thus are in strong disagreement with the widely accepted hypothesis that the global deep-sea floor became severely food-depleted following the K/Pg extinction due to the mass extinction of primary producers ("Strangelove Ocean Model") or to the collapse of the biotic pump ("Living Ocean Model").

Palaeogeography Palaeoclimatology Palaeoecology, 2007

Science, 2010

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Paleoceanography. 27, 1-14. doi:10.1029/2012PA002300, 2012

GSA Bulletin. 122(9/10), 1616-1624., 2010

The global warming and major perturbation of the global carbon cycle that occurred during the Pal... more The global warming and major perturbation of the global carbon cycle that occurred during the Paleocene-Eocene Thermal Maximum (PETM) have been investigated in the lower bathyal-upper abyssal Alamedilla section (Spain). Geochemical anomalies and dramatic faunal changes (including the globally recognized extinction event of deep-sea benthic foraminifera and the rapid evolutionary turnover of planktic foraminifera and calcareous nannofossils) are associated with the PETM at Alamedilla.

Marine Geology. 282, 178-187., 2011

Trace fossil assemblages from the latest Paleocene to the earliest Eocene were significantly affe... more Trace fossil assemblages from the latest Paleocene to the earliest Eocene were significantly affected by the environmental perturbation of the Paleocene-Eocene Thermal Maximum (PETM). High-resolution ichnological analysis shows well marked different ichnological features pre-, syn-, and post-event. A well developed normal, tiered burrowing community is present in the sediments below the PETM, indicating oxic conditions and normal benthic food availability. During the PETM the record of trace fossil producers disappeared gradually but rapidly, reflecting the global increase in temperature, and the concentration of benthic food in the very shallow surface layer and, probably, the local depletion of oxygen within the sediments, although probably not true anoxia. The environmental perturbation significantly affected the whole benthic habitat, as shown by the correspondence with the main phase of the benthic foraminiferal extinction. After the PETM, the normal, tiered burrowing community recovered gradually and slowly, in a delayed return to pre-PETM environmental conditions. The changes in the trace fossil assemblage thus document the impact of the PETM on the macrobenthic community, a decline in oxygen levels during the PETM in a globally perturbed habitat due to global warming and the similarities and differences in the response of micro-and macrobenthic communities to global phenomena. Thus, ichnological analysis reveals as a very useful additional tool to understanding atmosphere-ocean dynamic during PETM and a potential way in future climate research.

Journal of Foraminiferal Research. 42(4), 286-304., 2012

The effects of Oligocene paleoclimatic and paleoenvironmental events at lower latitudes have not ... more The effects of Oligocene paleoclimatic and paleoenvironmental events at lower latitudes have not been well defined, and the timing and extent of a proposed warming period in the late Oligocene are not clear. The study of benthic foraminifera from the upper bathyal Fuente Caldera section in southern Spain may help reconstruct the Oligocene paleoenvironmental turnover in the western Tethys. Rupelian and Chattian sediments from Fuente Caldera consist of hemipelagic marls intercalated with turbiditic sandstones. Based on a closely spaced sample collection, we present a quantitative analysis of benthic foraminiferal assemblage changes, and a detailed taxonomic study of 19 of the most abundant and paleoenvironmentally important species, belonging to the asterigerinids, rosalinids and bolivinids.

Episodes. 34(2), 86-108., 2011

Yes. Estimated paleo-sea depth is 1500 m; being located at a paleo-latitude of 35ºN, biostratigra... more Yes. Estimated paleo-sea depth is 1500 m; being located at a paleo-latitude of 35ºN, biostratigraphic correlations (open correlations with both Boreal and Tethysian pelagic regions are possible. The marine environment).

Episodes. 34(4), 220-243., 2011

220 Articles Episodes Vol. 34, no. 4 221 interval associated with the Mid-Paleocene Biotic Event,... more 220 Articles Episodes Vol. 34, no. 4 221 interval associated with the Mid-Paleocene Biotic Event, and it corresponds to the base of magnetochron C26n in the section. The base of the Thanetian is not associated with any significant change in marine micro-fauna or flora. The calcareous nannofossil Zone NP6, marked by the first occurrence of Heliolithus kleinpelli starts ca. 6.5 m below the base of the Thanetian. The definitions of the global stratotype points for the bases of the Selandian and Thanetian stages are in good agreements with the definitions in the historical stratotype sections in Denmark and England, respectively.

III Congreso Ibérico de Paleontología, Lisboa. 118-121., 2010

Eventos climáticos con foraminíferos bentónicos del tránsito Oligoceno -Mioceno en la sección de ... more Eventos climáticos con foraminíferos bentónicos del tránsito Oligoceno -Mioceno en la sección de Zarabanda (Cordilleras Béticas, España) Climatic events based on benthic foraminifera from the Oligocene -Miocene transition from the Zarabanda section (Betic Cordillera, Spain) Resumen: La sección de Zarabanda (Cordilleras Béticas, España) nos ha permitido estudiar el tránsito Oligoceno -Mioceno. El análisis cuantitativo de las asociaciones de los foraminíferos bentónicos proporciona información relevante sobre los cambios paleoambientales y paleoclimáticos. Los foraminíferos bentónicos indican una profundidad de depósito batial inferior (1000 m) para gran parte de esta sección. Además, se han reconocido dos probables caídas del nivel mar relacionadas con dos eventos globales de glaciación, los eventos Oi-2c y Mi-1. Palabras clave: Foraminíferos bentónicos, paleoclimatología, paleoambiente, Oligoceno-Mioceno. Abstract: The Zarabanda section (Betic Cordillera, Spain) allowed us to study the Late Oligocene-Lower Miocene transition. The quantitative analysis of benthic foraminiferal assemblages provided us with relevan! information about paleoenvironmental and paleoclimatological changes. Benthic foraminifera indica/e a lower bathyal (1000 m) depth of deposition for most par! of the studied section. Moreover, we identified two sea-leve! falls that might be related to two global glaciation events, namely the Oi-2c and Mi-1 events.

Geogaceta. 49, 23-26., 2010

Cenomanian/Turonian (C/T) boundary, coinciding with the Oceanic Anoxic Event 2 (OAE2) or Bonarell... more Cenomanian/Turonian (C/T) boundary, coinciding with the Oceanic Anoxic Event 2 (OAE2) or Bonarelli Event. We present an interdisciplinary analysis of the C/T transition at the Spanish El Chorro section (Betic Cordillera), and show the results of studies on planktic and benthic foraminifera, calcareous nannoplankton, radiolaria and environmental magnetic properties. The planktic foraminiferal biozones Rotalipora cushmani, Whiteinella archaeocretacea and Helvetoglobotruncana helvetica and the calcareous nannoplankton biozones NC11, NC12 and NC13 have been recognized. Diversity of the radiolarian assemblages gradually decreases from the upper Cenomanian towards the middle part of the OAE2 interval, where it reaches the lowest values, recovering above the OAE2. Oxygenation of the sea-bottom waters decreased towards the late Cenomanian, as inferred from a decrease in species size of benthic foraminifera 40 cm below the OAE2. Assemblages from the upper part of the section contain abundant and tiny epifaunal, trochospiral species and representatives of Tappanina, Pleurostomella and small buliminids. These data indicate that low oxygen conditions persisted towards the top of the section, 370 cm above the top of the OAE2 black shales.

Palaeogeography, Palaeoclimatology, Palaeoecology. 279, 186-200., 2009

A complete succession of lower bathyal-upper abyssal sediments was deposited across the Paleocene... more A complete succession of lower bathyal-upper abyssal sediments was deposited across the Paleocene-Eocene Thermal Maximum (PETM) at Alamedilla (Betic Cordillera, Southern Spain), where the benthic foraminiferal turnover and extinction event associated with the negative carbon isotope excursion (CIE) across the PETM have been investigated. Detailed quantitative analyses of benthic foraminifera allowed us to distinguish assemblages with paleoecological and paleoenvironmental significance: pre-extinction fauna, extinction fauna, survival fauna (including disaster and opportunistic fauna) and recovery fauna. These assemblages have been associated with significant parts of the δ 13 C curve for which a relative chronology has been established. The correlation between the benthic turnover, the δ 13 C curve, the calcite and silicate mineral content, and sedimentation rates, allowed us to establish the sequence of events across the PETM. At Alamedilla, the benthic extinction event (BEE) affected~37% of the species and it has been recorded over a 30-cm-thick interval that was deposited in c.a. 10 ky, suggesting a gradual but rapid pattern of extinction. The beginning of the BEE coincides with the onset of the CIE (+ 0 ky) and with an interval with abundant calcite, and it has been recorded under oxic conditions at the seafloor (as inferred from the benthic foraminiferal assemblages and the reddish colour of the sediments). We conclude that dissolution and dysoxia were not the cause of the extinctions, which were probably related to intense warming that occurred before the onset of the CIE. The BEE is immediately overlain by a survival interval dominated by agglutinated species (the Glomospira Acme). The low calcite content recorded within the survival interval may result from the interaction between dilution of the carbonate compounds by silicicate minerals (as inferred from the increased sedimentation rates), and the effects of carbonate dissolution triggered by the shoaling of the CCD. We suggest that Glomospira species (disaster fauna) may have bloomed opportunistically in areas with methane dissociation, in and around the North Atlantic. The disaster fauna was rapidly replaced by opportunistic taxa, which point to oxic and, possibly, oligotrophic conditions at the seafloor. The CCD gradually dropped during this interval, and calcite preservation improved towards the recovery interval, during which the δ 13 C values and the calcite content recovered (c.a. + 71.25 to 94.23 ky) and stabilized (N94.23 ky), coeval with a sharp decrease in sedimentation rates.

Episodes. 32(2), 84-95., 2009

The Global Stratotype Section and Point (GSSP) for the Cretaceous/Paleogene (KlPg) boundary was d... more The Global Stratotype Section and Point (GSSP) for the Cretaceous/Paleogene (KlPg) boundary was defined at the base ofthe boundary clay at a section near El Kef, Tunisia, but the outcrop became quite deteriorated. In order to better characterize the boundary and to solve problems of correlalíon, several auxiliary seclíons are designed and described in detail including: Ai"n Sett(lra and Elles in Tunisia, Caravaca and Zumaya in Spain, Bidart in France and El Mulato and Bochil in Mexico. These sections are the most continuous, expanded and representative of marine sedimentation in areas proximal and distal to the Chicxulub meteorite impact site. In addilíon, these sections are classical, very well known, physical!y accessible, have been exhaustively studied and allow a very detailed global correlalíon. The correlalíon critería used were the meteorite impact evidence (Ir anomaly, Ni-rich spínel, etc.) and the mass extinclíon of planktíc mícro-and nannofossils. Furthermore, it was proposed that the KlPg boundary is marked exactly by the moment ofthe meteorite impact, which implies that al! the sediments generated by the impact belong to the Haleogene: While in distal areas to the impact site the KlPg boundary coincides with a millimetre-thick rusty layer, in proximal areas the KlPg boundary correlates to the base of a metre-thick Clastic Unit, including a thick calcareous breccia in the sections closer to the impact crater.

Journal of Iberian Geology. 31(1), 135-148., 2005

The Cretaceous/Paleogene (K/Pg) boundary event has been extensively studied in the Spanish Agost ... more The Cretaceous/Paleogene (K/Pg) boundary event has been extensively studied in the Spanish Agost section, which contains one of the most continuous and expanded Cretaceous-Paleogene transitions in the Tethys area. For that reason, it is considered as a classical K/Pg boundary section, and numerous researchers have carefully analysed it from different points of view, such as micropaleontology, paleoichnology, magnetostratigraphy, mineralogy and geochemistry. Sediments from the Upper Cretaceous (Abathomphalus mayaroensis and Plummerita hantkeninoides planktic foraminiferal Biozones), and from the lower Paleogene (Guembelitria cretacea, Parvularugoglobigerina eugubina, Parasubbotina pseudobulloides and Globanomalina compressa Biozones), correspond to a marly, microfossil-rich sequence that was deposited in the upper and middle part of the slope, as indicated by the benthic foraminiferal assemblages. A dark clay layer containing impact evidence is identified at the base of the Danian. Planktic foraminifera show a catastrophic mass extinction pattern in coincidence with the K/Pg, just at the base of the dark clay layer. Approximately 70% of the species clearly became extinct at the K/Pg boundary. Very few species seem to disappear in the uppermost Maastrichtian, although these disappearances might result from the background extinction pattern or the remaining Signor-Lipps effect. Some Cretaceous species seem to have survived, and gradually disappear during the Danian, although this might be due to the long-term effect of the asteroid impact; their presence in Danian sediments can be also interpreted as the result of reworking processes. If we take this into account, the percentage of planktic foraminifera that became extinct at the K/Pg boundary makes up to 90% of the species. In contrast to planktic foraminifera, benthic foraminifera did not suffer any mass extinction, although the drastic reorganization of their assemblages in coincidence with the boundary reflects important environmental changes. These changes are compatible with the castastrophic effects of a large asteroid impact that occurred just at the Cretaceous/Paleogene boundary.