Alfredo Martínez-García

Max Planck Institute for Chemistry | mpic · Department of Climate Geochemistry

Most anthropogenic nitrogen (N) reaches coastal waters via rivers carrying increasing loads of sewage, fertilizer, and sediments. To understand anthropogenic N impacts, we need to understand historical N-dynamics before human influence. Stable isotope ratios of N preserved in carbonates are one way to create temporal N records. However, records tha...

Studies have suggested that cancerous tissue has a lower ¹⁵N/¹⁴N ratio than benign tissue. However, human data have been inconclusive, possibly due to constraints on experimental design. Here, we used high-sensitivity nitrogen isotope methods to assess the ¹⁵N/¹⁴N ratio of human breast, lung, and kidney cancer tissue at unprecedented spatial resolu...

Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial-interglacial climate cycles. However, constraining past Agulhas leakage remains a challenge. Using new measurements from the modern South Atlantic, we propose that the δ15N of organic matter pres...

Millennial-scale climate variations during the last glacial period, such as Dansgaard–Oeschger (D/O) cycles and Heinrich events, have been extensively studied using ice core and marine proxy records. However, there is a limited understanding of the magnitude of these temperature fluctuations in continental regions, and questions remain about the se...

There is limited understanding of temperature and atmospheric circulation changes that accompany an Atlantic Meridional Overturning Circulation (AMOC) slowdown beyond the North Atlantic realm. A Peqi'in Cave (Israel) speleothem dated to the last interglacial period (LIG), 129-116 thousand years ago (ka), together with a large modern rainfall monito...

The ability of scleractinian corals to thrive in the oligotrophic (low-nutrient, low-productivity) surface waters of the tropical ocean is commonly attributed to their symbiotic relationship with photosynthetic dinoflagellates 1,2 . The evolutionary history of this symbiosis might clarify its organismal and environmental roles. It is currently uncl...

The oxygen content of the oceans is susceptible to climate change and has declined in recent decades¹, with the largest effect in oxygen-deficient zones (ODZs)², that is, mid-depth ocean regions with oxygen concentrations <5 μmol kg⁻¹ (ref. ³). Earth-system-model simulations of climate warming predict that ODZs will expand until at least 2100. The...

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs; however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotope composition of organic matter preserved in mammalian tooth enamel (δ¹⁵Nenamel) r...

The greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters...

It is understood that the global mean ocean nitrate δ¹⁵N is set by the δ¹⁵N of the input of fixed nitrogen (N) to the ocean (mostly N2 fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate assimi...

The proposed Anthropocene Global Boundary Stratotype Section and Point (GSSP) candidate site of West Flower Garden Bank (27.8762°N, 93.8147°W) is an open ocean location in the Gulf of Mexico with a submerged coral reef and few direct human impacts. Corals contain highly accurate and precise (<±1 year) internal chronologies, similar to tree rings, a...

Corals are unique in the suite of proposed Anthropocene Global Boundary Stratotype Section and Point (GSSP) archives, as living organisms that produce aragonite exoskeletons preserved in the geological record that contain highly accurate and precise (<±1 year) internal chronologies. The GSSP candidate site North Flinders Reef in the Coral Sea (Aust...

Corals are unique in the suite of proposed Anthropocene Global Boundary Stratotype Section and Point (GSSP) archives, as living organisms that produce aragonite exoskeletons preserved in the geological record that contain highly accurate and precise (<±1 year) internal chronologies. The GSSP candidate site North Flinders Reef in the Coral Sea (Aust...

Along the coastal Tropical Eastern Pacific (TEP), regions of strong seasonal upwelling bring cold, nutrient-rich waters, controlling ecological conditions and sustaining millions of people through large-scale fisheries. The TEP is also important for the regulation of global climate and is affected by large-scale environmental processes such as ENSO...

The cyclic growth and decay of continental ice sheets can be reconstructed from the history of global sea level. Sea level is relatively well constrained for the Last Glacial Maximum (LGM, 26,500 to 19,000 y ago, 26.5 to 19 ka) and the ensuing deglaciation. However, sea-level estimates for the period of ice-sheet growth before the LGM vary by > 60...

Lake sediment records from Holzmaar and the infilled maar of Auel (Eifel, Germany) are used to reconstruct landscape changes and megafauna abundances. Our data document a forested landscape from 60,000 to 48,000 yr b2k and a stepwise vegetation change towards a glacial desert after 26,000 yr b2k. The Eifel landscape was continuously inhabited from...

The modern Pacific Ocean hosts the largest oxygen-deficient zones (ODZs), where oxygen concentrations are so low that nitrate is used to respire organic matter. The history of the ODZs may offer key insights into ocean deoxygenation under future global warming. In a 12-My record from the southeastern Pacific, we observe a >10‰increase in foraminife...

The analyses of the stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and oxygen (δ18O) in animal tissues are powerful tools for reconstructing the feeding behavior of individual animals and characterizing trophic interactions in food webs. Of these biomaterials, tooth enamel is the hardest, most mineralized vertebrate tissue and therefore l...

Dissolved oxygen (O 2 ) is essential for most ocean ecosystems, fuelling organisms’ respiration and facilitating the cycling of carbon and nutrients. Oxygen measurements have been interpreted to indicate that the ocean’s oxygen-deficient zones (ODZs) are expanding under global warming 1,2 . However, models provide an unclear picture of future ODZ c...

Fossil-bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil-bound N isotopic composition of different fossil types, including deep an...

Nitrogen isotopes are widely used to study the trophic position of animals in modern food webs, however, their application in the fossil record is severely limited by degradation of organic material during fossilization. In this study, we show that the nitrogen isotopic composition of organic matter preserved in mammalian tooth enamel (δ15Nenamel)...

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the 15N/14N ratio of enameloid-bound organic matter (δ15NEB) as a trophic level proxy by comparison to dentin collagen δ15N and apply this method to the fossil record to reconstruct the trophic level of the megatoot...

The Southern Hemisphere Westerly Winds (SWW) drive upwelling south of the Antarctic Polar Front (PF) that vents CO2 to the atmosphere. During the ice ages, a northward (equatorward) shift of the PF may have reduced this CO2 venting, helping to explain the lower atmospheric CO2 concentration of those times. However, evidence of PF migration is lacki...

During glacial terminations, massive iceberg discharges and meltwater pulses in the North Atlantic triggered a shutdown of the Atlantic Meridional Overturning Circulation (AMOC). Speleothem calcium carbonate oxygen isotope records (δ ¹⁸ O Cc ) indicate that the collapse of the AMOC caused dramatic changes in the distribution and variability of the...

Ocean circulation supplies the surface ocean with the nutrients that fuel global ocean productivity. However, the mechanisms and rates of water and nutrient transport from the deep ocean to the upper ocean are poorly known. Here, we use the nitrogen isotopic composition of nitrate to place observational constraints on nutrient transport from the So...

Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present foraminifera...

During the last ice age, the Northern Hemisphere experienced a series of abrupt millennial-scale climatic changes linked to variations in the strength of the Atlantic Meridional Overturning Circulation and sea-ice extent. However, our understanding of their impacts on decadal-scale climate variability in central Europe has been limited by the lack...

Background Cancerous cells can recycle metabolic ammonium for their growth. As this ammonium has a low nitrogen isotope ratio ( ¹⁵ N/ ¹⁴ N), its recycling may cause cancer tissue to have lower ¹⁵ N/ ¹⁴ N than surrounding healthy tissue. We investigated whether, within a given tissue type in individual mice, tumoral and healthy tissues could be dist...

Downcore sediment grain-size records of mineral dust (2–10 μm) can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about glacial-interglacial changes of dust grain size in the open Southern Ocean, which are potentially associated with changes i...

Shells of oysters (Ostreidae) are predominantly composed of foliated and chalky calcite microstructures. The formation process of the more porous chalky structure is subject to debate, with some studies suggesting that it is not formed directly by the oyster but rather through microbial mineralization within the shell. Here, this hypothesis is test...

Bulk sediment δ¹⁵N records from the eastern tropical Pacific (ETP) extending back to the last ice age most often show low glacial δ¹⁵N, then a deglacial δ¹⁵N maximum, followed by a gradual decline to a late Holocene δ¹⁵N that is typically higher than that of the Last Glacial Maximum (LGM). The lower δ¹⁵N of the LGM has been interpreted to reflect a...

The Middle Pleistocene Transition (MPT) represents a major change in Earth's climate state, exemplified by the switch from obliquity-dominated to ∼100-kyr glacial/interglacial cycles. To date, the causes of this significant change in Earth's climatic response to orbital forcing are not fully understood. Nonetheless, this transition represents an in...

Methods for reconstructing past temperatures from speleothems have only recently been developed. Advances in quantitative temperature proxies for speleothems are now allowing critical knowledge gaps to be filled, given the outstanding age control and wide geographical distribution of the speleothem archive. The methods of reconstructing temperature...

For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The δ¹³C of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water. Relative t...

The modern Pacific Ocean hosts the largest oxygen-deficient zones (ODZs), where oxygen concentrations are so low that nitrate is used to respire organic matter. However, the history of these ODZs is not well understood, hampering a mechanistic prediction of how these ODZs might evolve in the future under global warming. In a 12-million-year (Ma) re...

Controlling atmospheric carbon dioxide The atmospheric concentration of carbon dioxide (CO 2 ) has varied substantially over the past million years in tandem with the glacial cycle. Although it is widely agreed that upwelling of Southern Ocean water is a key factor, the finer details about what caused these CO 2 variations are of great importance f...

Past changes in ocean ¹⁴C disequilibria have been suggested to reflect the Southern Ocean control on global exogenic carbon cycling. Yet, the volumetric extent of the glacial carbon pool and the deglacial mechanisms contributing to release remineralized carbon, particularly from regions with enhanced mixing today, remain insufficiently constrained....

The Southern Ocean is widely recognized as a potential cause of the lower atmospheric concentration of CO2 during ice ages, but the mechanism is debated. Focusing on the Southern Ocean surface, we review biogeochemical paleoproxy data and carbon cycle concepts that together favor the view that both the Antarctic and Subantarctic Zones (AZ and SAZ)...

Nitrogen isotope ratios (δ¹⁵N) are a well-established tool for investigating the dietary and trophic behavior of animals in terrestrial and marine food webs. To date, δ¹⁵N values in fossils have primarily been measured in collagen extracted from bone or dentin, which is susceptible to degradation and rarely preserved in deep time (>100,000 years)....

Over the last deglaciation there were two transient intervals of pronounced atmospheric CO2 rise; Heinrich Stadial 1 (17.5-15 kyr) and the Younger Dryas (12.9-11.5 kyr). Leading hypotheses accounting for the increased accumulation of CO2 in the atmosphere at these times invoke deep ocean carbon being released from the Southern Ocean and an associat...

The middle Miocene climate transition (~14 million years ago) was characterized by a dramatic increase in the volume of the Antarctic ice sheet. The driving mechanism of this transition remains under discussion, with hypotheses including circulation changes, declining carbon dioxide in the atmosphere and orbital forcing. Southern Ocean records of p...

A method is described for simultaneous extraction and chromatographic separation of saturated hydrocarbons (n-alkanes) and unsaturated long-chain ketones (alkenones) from glycerol dialkyl glycerol tetraethers (GDGTs) using selective accelerated solvent extraction (ASE). We show that the method can be used to successfully separate n-alkanes and alke...

We present the first nitrogen isotope (δ15N) measurements of planktic foraminifera, paleoceanographically important zooplankton, from the nutrient‐rich waters of the modern Southern Ocean. Foraminifera were collected from net tows in the Subantarctic and Polar Frontal Zones (SAZ and PFZ, respectively) south of Africa during winter 2015 and late sum...

Sabkhas are important settings for understanding early earth biological environments, and the algal mats associated with them are thought to be potential source rocks for hydrocarbon production. In this study we compare the sedimentological facies and distribution of branched, and isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) in modern a...

The Indonesian Throughflow (ITF) controls the oceanic flux of heat and salt between the Pacific and Indian Oceans and therewith plays an important role in modulating the meridional overturning circulation and low latitude hydrological cycle. Here, we report new sea surface temperature and aridity records from the west coast of Australia (IODP Site...

Historical coral skeleton (CS) δ18O and δ15N records were produced from samples recovered from sedimentary deposits, held in natural history museum collections and cored into modern coral heads. These records were used to assess the influence of global warming and regional eutrophication, respectively, on the decline of coastal coral communities fo...

We present ²³⁰Th-normalized dust and export production fluxes for two contrasted marine sediment cores spanning the Antarctic Polar Front, close to the Kerguelen Plateau in the Southern Indian Ocean, covering the last glacial cycle. We report glacial lithogenic fluxes comparable to the South Atlantic and higher than in the South Pacific sectors of...

The Pliocene was a globally-warm high-CO2 epoch. Yet, four globally-recognized glacial events interrupted the relatively warm climate conditions at 4.9, 4.0, 3.7-3.6 and 3.3 Ma. The Indonesian Throughflow (ITF) is an important link in the global thermohaline circulation and is hypothesised to amplify these spells of increased climate variability. Y...

The shoaling of the Central American Seaway (CAS) around 4.6 Ma (million years ago) is thought to have enhanced the Gulf Stream, strengthening the Atlantic Meridional Overturning Circulation and potentially influencing the evolution of Pliocene climate. Paleoclimate records indicate a buildup of heat and salt in the Caribbean and changes in the for...

Circulation more than temperature Changes in continental configuration and sea level affected the ocean's oxygen levels and the rate of denitrification between 70 and 30 million years ago. That finding by Kast et al. shows a fundamental difference from the modern ocean, in which the extent of suboxia is controlled primarily by global temperature. C...

Resetting the glacial timer The periodicity of glacial cycles changed from 100,000 to 41,000 years during the middle of the Pleistocene epoch. Why? Hasenfratz et al. measured the oxygen isotope composition and magnesium/calcium ratio in benthic and planktonic foraminifera from the Antarctic in order to reconstruct changes in the rate of transfer of...

Both the nitrogen (N) isotopic composition (δ^(15)N) of the nitrate source and the magnitude of isotope discrimination associated with nitrate assimilation are required to estimate the degree of past nitrate consumption from the δ^(15)N of organic matter in Southern Ocean sediments (e.g., preserved within diatom microfossils). It has been suggested...

High‐resolution palaeorecords of climate are critical to improving current understanding of climate variability, its sensitivity and impact on the environment in the past and in the future. Sediments from the Cariaco Basin off the coast of Venezuela have proven to be sensitive recorders of tropical palaeoclimate variability down to an annual scale....

In the Antarctic Zone of the Southern Ocean, the coupled observations of elevated diatom-bound ¹⁵N/¹⁴N (δ¹⁵Ndb) and reduced export production during the ice ages indicates more complete nitrate (NO3⁻) consumption. This evidence points to an ice age decline in gross NO3⁻ supply from the deep ocean to the surface wind-mixed layer, which may help to e...

Ocean dynamics served an important role during past dramatic climate changes via impacts on deep-ocean carbon storage. Such changes are recorded in sedimentary proxies of hydrographic change on continental margins, which lie at the ocean–atmosphere–earth interface. However, interpretations of these records are challenging, given complex interplays...

A rise in the atmospheric CO2 concentration of ~20 parts per million over the course of the Holocene has long been recognized as exceptional among interglacials and is in need of explanation. Previous hypotheses involved natural or anthropogenic changes in terrestrial biomass, carbonate compensation in response to deglacial outgassing of oceanic CO...